Light and Optics — Free Physics Review Games.

Light is an electromagnetic wave that can travel through a vacuum without needing a medium.

Light travels at approximately 3 x 10^8 meters per second (300,000 km/s) in a vacuum.

Reflection occurs when light bounces off a surface, following the law of reflection.

Refraction is the bending of light when it passes between media of different densities due to a change in speed.

Red light has the longest wavelength in the visible spectrum (about 700 nm), while violet has the shortest.

The law of reflection states that the angle of incidence equals the angle of reflection, measured from the normal.

A convex (converging) lens is thicker in the middle and bends light rays toward a focal point.

The electromagnetic spectrum encompasses all electromagnetic radiation: radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma ray.

Rainbows form when sunlight is refracted, dispersed by wavelength, and internally reflected inside water droplets.

A concave (converging) mirror curves inward and can focus parallel light rays to a focal point.

Total internal reflection occurs when light traveling in a denser medium hits the boundary at an angle greater than the critical angle, reflecting entirely back.

Diffraction is the bending of waves around obstacles or through openings, most noticeable when the opening is comparable to the wavelength.

The index of refraction (n) indicates how much light slows down in a medium: n = c/v, where c is the speed of light in vacuum.

Using the mirror equation: 1/f = 1/do + 1/di. 1/10 = 1/30 + 1/di. 1/di = 1/10 - 1/30 = 2/30 = 1/15. di = 15 cm.

A prism causes dispersion because different wavelengths of light refract at slightly different angles due to varying speeds in the glass.

The normal is defined as the line perpendicular to the surface at the point where the light ray strikes, and all angles of incidence, reflection, and refraction are measured from it. The choice "A line parallel to the surface" is wrong because a line parallel to the surface would coincide with the surface itself and could not serve as a reference for measuring angles. Students should always remember that angles in optics diagrams are measured from the normal, never from the surface.

Gamma rays sit at the far end of the electromagnetic spectrum with the shortest wavelengths, and since energy is inversely proportional to wavelength (\(E=hc/\lambda\)), they carry the highest photon energy. "Radio waves" is incorrect because radio waves have the longest wavelengths and thus the lowest energy of all EM radiation. On the exam, remember the ordering of the spectrum: as wavelength decreases, frequency and energy increase.

A virtual image forms where diverging light rays appear to originate when traced backward, but no actual light passes through that point, so it cannot be captured on a screen. The distractor "An image formed where light rays physically converge and can be projected onto a screen" describes a real image, the opposite concept. Students should learn to distinguish virtual images (like those in a flat mirror or from a diverging lens) from real images that can be projected.

A real image occurs when refracted or reflected light rays actually meet at a point, allowing the image to be displayed on a screen such as film or a wall. The option "An image that light rays only appear to come from" describes a virtual image, not a real one. A key exam principle is that real images can be projected while virtual images cannot.

A convex mirror curves outward, so parallel incoming rays reflect outward from a virtual focal point behind the mirror, causing the rays to diverge. "Concave mirror" is incorrect because a concave mirror curves inward and converges parallel rays toward a real focal point in front of it. Remembering the curvature direction of a mirror tells you immediately whether it converges or diverges light.

Light slows down when entering a denser medium like glass because the higher index of refraction of glass corresponds to a lower speed, following \(n=c/v\). The option "It increases" is wrong because speed only increases when light moves into a less dense medium, not a denser one. A core principle is that the index of refraction of a medium directly determines how much light slows down inside it.

The focal point is defined as the location where incoming parallel light rays meet after being refracted by a converging lens or reflected by a concave mirror. "The point exactly at the center of curvature" is incorrect because the center of curvature is a separate point, located at twice the focal length from the mirror's surface. Students should remember the focal point is a fixed optical property of the lens or mirror, independent of where the object happens to be placed.

Violet light sits at the short-wavelength end of the visible spectrum, around 380-450 nm, making it the shortest wavelength color humans can see. "Red" is incorrect because red light has the longest wavelength in the visible spectrum, roughly 620-750 nm. A useful memory principle is that visible colors run from long wavelength red to short wavelength violet, often remembered with the acronym ROYGBIV.

An opaque material absorbs and/or reflects incident light but does not allow it to pass through to the other side, which is why you cannot see through opaque objects. The choice "It allows all light to pass through unchanged" actually describes a transparent material, the opposite behavior. Students should keep the three categories straight: transparent materials transmit light clearly, translucent materials scatter it, and opaque materials block it entirely.

Frequency measures the number of wave cycles per second, and its SI unit is the hertz (\(Hz\)), equivalent to \(1/s\). "Meters (\(m\))" is incorrect because meters measure wavelength, a spatial distance, not a rate of oscillation. Remember that wavelength and frequency are related by \(c=\lambda f\), but they are measured in different units entirely.

A concave lens is thinner in the middle than at the edges, which causes parallel rays entering it to bend outward and diverge as if coming from a virtual focal point on the incoming side. "Convex (converging) lens" is wrong because a convex lens is thicker in the middle and bends parallel rays inward toward a real focal point. Recognizing lens shape (thicker vs. thinner in the middle) is the quickest way to predict whether it converges or diverges light.

The angle of incidence is specifically measured between the incoming light ray and the normal, the perpendicular reference line at the point of contact. "The angle between the incoming ray and the surface" is incorrect because that would be the complement of the true angle of incidence, not the standard optics definition. Always measure incidence, reflection, and refraction angles relative to the normal, a convention used throughout optics.

Radio waves occupy the low-frequency, long-wavelength end of the electromagnetic spectrum, extending to wavelengths of meters or even kilometers. "Ultraviolet light" is incorrect because ultraviolet has a much shorter wavelength than radio waves and sits closer to the visible and X-ray regions. Knowing the full spectrum order from radio to gamma rays helps you quickly rank wavelength, frequency, and energy on exam questions.

The law of reflection states that the angle of incidence equals the angle of reflection, so a \(35^\circ\) incident angle produces a \(35^\circ\) reflected angle measured from the same normal. The distractor \(55^\circ\) is incorrect because it represents the complement of the incidence angle relative to the mirror surface rather than the normal. Whenever a problem gives an angle from the normal, apply the law of reflection directly without needing extra calculation.

When the object distance exceeds \(2F\), the converging lens forms a real, inverted image located between \(F\) and \(2F\) on the opposite side, and this image is smaller than the object because the object is farther from the lens than the image. The option "A virtual, upright image that is larger than the object" describes what happens when the object is placed inside the focal length instead, not beyond \(2F\). Students should memorize how image type, orientation, and size change as an object moves through the key zones (\(F\), \(2F\)) relative to a converging lens.

Snell's law, \(n_1\sin\theta_1 = n_2\sin\theta_2\), requires placing the incident medium's index (\(1.00\) for air) with the incidence angle and the refracted medium's index (\(1.33\) for water) with the refraction angle. The option \((1.33)\sin(40^\circ) = (1.00)\sin\theta_2\) is incorrect because it swaps which index belongs to which angle, reversing the physical setup. Always identify which medium the light is leaving and which it is entering before applying Snell's law.

Substituting into the mirror equation gives \(\frac{1}{15}=\frac{1}{30}+\frac{1}{d_i}\), so \(\frac{1}{d_i}=\frac{1}{15}-\frac{1}{30}=\frac{1}{30}\), meaning \(d_i=30\,cm\). The option \(15\,cm\) is wrong because it mistakenly equates the image distance to the focal length rather than solving the rearranged equation. This algebraic manipulation of the mirror equation is essential and should be practiced until it becomes automatic.

Magnification is calculated as the image height divided by the object height, so \(m = \frac{12\,cm}{4\,cm} = 3\), meaning the image is three times the size of the object. The option \(0.33\) is incorrect because it inverts the ratio, dividing object height by image height instead of the correct order. Remember that magnification greater than 1 indicates an enlarged image, while magnification less than 1 indicates a reduced image.

The correct order from shortest to longest wavelength follows the standard spectrum: X-rays are shortest, then ultraviolet, then visible light, and microwaves have the longest wavelength among this group. The option "Microwaves, visible light, ultraviolet, X-rays" is exactly reversed, listing the longest wavelength first instead of last. Memorizing the full spectrum sequence from gamma rays to radio waves lets you quickly order any subset of EM waves by wavelength.

Because light slows down when entering a denser medium, the ray bends toward the normal to satisfy Snell's law, which requires a smaller angle on the higher-index side. The option "It bends away from the normal line" describes what happens when light moves from a denser to a less dense medium instead. A helpful rule to remember is: slower medium, smaller angle, bend toward the normal; faster medium, larger angle, bend away.

A plane mirror produces a virtual image because reflected rays only appear to diverge from behind the mirror, and this image is always upright and the same size as the object due to the flat, non-curved geometry. The option "Real, inverted, and larger than the object" describes a typical curved-mirror image, not one from a flat mirror. A key fact to remember is that plane mirrors never magnify or invert images, unlike curved mirrors.

By convention, converging lenses are given a positive focal length because they bring light rays together on the real-image side of the lens. The option "It is assigned a negative value" is incorrect because negative focal lengths are reserved for diverging lenses, which spread light apart. Keeping sign conventions straight (positive for converging, negative for diverging) prevents errors when solving the thin lens equation.

The shirt's pigment absorbs most wavelengths of the incident white light but reflects the red wavelengths, and those reflected photons reach your eye, giving the shirt its red appearance. The option "transmits red wavelengths and absorbs the rest" is incorrect because an opaque object does not transmit light through itself at all. Remember that the color you perceive from an opaque object is the wavelength it reflects, not the wavelengths it absorbs.

Glass has a slightly different index of refraction for each wavelength of light, so shorter wavelengths like violet bend more than longer wavelengths like red, spreading the colors apart in a process called dispersion. The option "All wavelengths travel at different speeds in a vacuum" is wrong because in a vacuum every wavelength of light travels at exactly the same speed, \(c\). The wider principle is that dispersion arises specifically from a medium's wavelength-dependent index of refraction, not from any inherent speed difference in empty space.

The critical angle is defined as the specific incidence angle in the denser medium where the refracted ray grazes exactly along the boundary at \(90^\circ\), marking the threshold beyond which total internal reflection occurs. The option "the light is completely absorbed by the medium" is incorrect because absorption is unrelated to the geometric condition that defines the critical angle. Remember that any incidence angle greater than the critical angle results in total internal reflection instead of refraction.

Placing the object between \(F\) and \(2F\) causes the converging lens to form a real, inverted image located beyond \(2F\) on the opposite side, and because the image distance exceeds the object distance in this zone, the image is magnified. The option "A virtual, upright image that is larger than the object" describes the case when the object is inside the focal length, not between \(F\) and \(2F\). Mastering these three object-position zones for a converging lens is essential for predicting image characteristics on the exam.

Microwaves have a wavelength that resonates with water molecules, causing them to absorb the energy and heat up food, while their wavelength also allows them to reflect off metal surfaces, which is exploited in radar detection. The option "Their extremely high energy allows them to ionize atoms" is incorrect because microwaves are non-ionizing radiation, unlike X-rays or gamma rays. A useful exam fact is that all EM waves travel at the same speed \(c\) in a vacuum, so speed differences never explain their differing practical uses.

X-rays occupy a region of the spectrum with shorter wavelengths than ultraviolet light, and since photon energy is given by \(E=hc/\lambda\), this shorter wavelength corresponds to higher energy per photon. The option "longer wavelength and lower photon energy" is incorrect because it reverses the actual spectral relationship between X-rays and ultraviolet light. Students should remember the inverse relationship between wavelength and photon energy across the entire electromagnetic spectrum.

Plane mirrors produce laterally inverted images, meaning left and right are swapped in the reflection even though top and bottom remain unchanged, so a raised right hand appears as a raised left hand in the mirror. The option "right hand, because mirrors preserve orientation" is incorrect because mirrors specifically do not preserve left-right orientation, only up-down orientation. This lateral inversion property is a classic feature of flat mirror reflection that is frequently tested.

Since \(n=c/v\), a larger index of refraction \(n\) corresponds mathematically to a smaller speed \(v\), meaning light travels more slowly through media with higher indices such as diamond compared to those with lower indices such as water. The option "speeds light up more than a medium with a lower index" is incorrect because it inverts the relationship described by the formula. This formula is central to solving both Snell's law problems and critical angle problems, so understanding its meaning is essential.

Warm objects naturally emit infrared radiation due to their thermal energy, and because infrared is invisible to human eyes, it can be used to send remote control signals without visible interference while also being detected by thermal cameras to map heat patterns. The option "it has higher energy than visible light and can penetrate metal" is incorrect because infrared actually has lower photon energy than visible light, not higher. Remembering that infrared relates directly to thermal emission is key to understanding its everyday applications.

A negative \(d_i\) signals that the image forms on the same side of the lens as the incoming light, which is characteristic of a virtual image that cannot be projected on a screen. The option "real and located on the opposite side of the lens from the object" is incorrect because that describes a positive \(d_i\), the sign used for real images. Always check the sign of a calculated image distance to determine whether the resulting image is real or virtual before describing its other properties.

Rearranging the thin lens equation gives \(\frac{1}{d_i}=\frac{1}{12}-\frac{1}{20}=\frac{5-3}{60}=\frac{1}{30}\), so \(d_i=30\,cm\), a real image beyond \(2F\) on the opposite side of the lens. The option \(7.5\,cm\) is incorrect because it results from adding rather than subtracting the reciprocals in the wrong order. This multi-step algebraic process of finding a common denominator is the most common source of error when applying the thin lens equation, so careful arithmetic matters.

Substituting the values gives \(\sin\theta_c = \frac{1.0}{1.5} \approx 0.667\), and taking the inverse sine yields \(\theta_c \approx 41.8^\circ\), the angle beyond which light inside the glass undergoes total internal reflection instead of exiting into air. The option \(48.6^\circ\) is incorrect because it results from inverting the ratio of indices in the critical angle formula. Total internal reflection only occurs when light moves from a higher-index medium toward a lower-index medium, a condition that must always be checked before applying this formula.

When an object sits exactly at the focal point, the thin lens equation gives an undefined (infinite) image distance because the refracted rays emerge parallel to each other and never converge or appear to diverge from a single point. The option "A real, inverted image forms exactly at \(2F\)" is incorrect because that outcome only occurs when the object is placed at \(2F\), not at \(F\). Recognizing this special boundary case at exactly \(F\) helps students understand why the object-position zones on either side produce dramatically different image types.

Rearranging the apparent depth formula gives real depth \(= \text{apparent depth} \times n = 0.75\,m \times 1.33 \approx 1.0\,m\), meaning the fish is actually deeper than it appears from above the water. The option \(0.56\,m\) is incorrect because it results from dividing by \(n\) instead of multiplying, reversing the correct algebraic step. This apparent-depth effect explains why objects underwater always look closer to the surface than they truly are when viewed from air.

Each plane mirror individually produces a laterally inverted image, but reflecting the light a second time off a parallel mirror reverses the inversion again, restoring the original left-right orientation for the viewer. The option "Plane mirrors never invert images regardless of number" is incorrect because a single plane mirror does laterally invert an image, as seen in ordinary mirror reflections. This principle of double reflection canceling inversion is why periscopes and similar two-mirror systems preserve correct orientation.

Since \(E=hc/\lambda\) shows energy is inversely proportional to wavelength, the shorter-wavelength blue photon carries more energy than the longer-wavelength red photon. The option "The blue photon has more energy because it travels faster than red light" is incorrect because both colors of visible light travel at the exact same speed \(c\) in a vacuum; only their energy and wavelength differ. This inverse energy-wavelength relationship applies across the entire electromagnetic spectrum, not just visible light.

Because the object distance (\(8\,cm\)) is less than the focal length (\(12\,cm\)), the converging lens acts like a magnifying glass, producing a virtual, upright, and enlarged image on the same side as the object. The option "Since the object is inside the focal length, the image will be real and inverted" is incorrect because real, inverted images only form when the object is placed beyond the focal length, not within it. Recognizing the object's position relative to \(F\) immediately tells you the general nature of the image before any calculation is even needed.

Substituting values gives \(\frac{1}{d_i} = \frac{1}{-15} - \frac{1}{20} = -\frac{7}{60}\), so \(d_i \approx -8.6\,cm\), and the negative sign confirms the image is virtual and located behind the mirror, consistent with how convex mirrors always form reduced, upright, virtual images. The option "approximately \(8.6\,cm\) in front of the mirror" is incorrect because it drops the negative sign, misrepresenting the image as real when convex mirrors never form real images from real objects. Remembering that convex mirrors always use a negative focal length by convention is essential for getting the sign of the result correct.

Optical fibers are engineered so that light entering the core strikes the boundary with the cladding at angles exceeding the critical angle, causing repeated total internal reflection that keeps the light confined and traveling along the fiber with little loss. The option "the glass core has zero index of refraction" is incorrect because every real material has a nonzero index of refraction, and a zero value is physically impossible. This application shows how the critical angle concept, usually taught abstractly, has a direct real-world engineering use in fiber-optic communication.

The slit spacing is \(d = \frac{1\,mm}{500} = 2000\,nm\), and since \(m=1\) for the first order, solving \(d\sin\theta = m\lambda\) gives \(\lambda = (2000\,nm)\sin(15^\circ) \approx 518\,nm\), a wavelength within the visible spectrum. The option \(\lambda = (500\,nm)\sin(15^\circ)\) is incorrect because it uses the number of lines per millimeter directly as the slit spacing instead of first converting it to the spacing in nanometers. Diffraction grating problems always require carefully converting line density into an actual spacing \(d\) before applying the grating equation.

When an object is placed closer to a concave mirror than its focal length, the reflected rays diverge after reflection and only appear to meet behind the mirror, producing a virtual, upright, and magnified image, similar to how a shaving or makeup mirror works. The option "A real, inverted image in front of the mirror" is incorrect because real images from a concave mirror only form when the object is placed beyond the focal length, not within it. Just as with converging lenses, the position of the object relative to \(F\) determines whether a concave mirror produces a real or virtual image.

Because the index of refraction of lens glass varies slightly with wavelength, different colors of light bend by different amounts and converge at slightly different focal points, producing color fringing known as chromatic aberration. The option "Chromatic aberration is caused by diffraction rather than refraction" is incorrect because this effect arises specifically from wavelength-dependent refraction (dispersion), not from diffraction, which is a separate wave phenomenon. This same wavelength-dependence of the index of refraction is also what causes prisms to spread white light into a spectrum of colors.

Polarization works by filtering out all oscillation directions except one plane, which is only possible for transverse waves like light, whose electric and magnetic fields oscillate perpendicular to the direction of travel; sound is a longitudinal wave that compresses and expands along its direction of travel, so it has no transverse oscillation to filter. The option "Light has mass while sound does not" is incorrect because light, as electromagnetic radiation, is massless, and mass is irrelevant to whether a wave can be polarized. The ability to polarize a wave is a defining test for whether it is transverse, a concept that distinguishes electromagnetic waves from mechanical longitudinal waves like sound.