Light: Shadows, Refraction, and Reflection

Comprehensive practice flashcards covering the fundamentals of optics: light propagation, point vs. extended sources, shadow geometry, solar and lunar eclipses, laws and applications of reflection, and refraction mechanics and calculations.

What is light, and how fast does it travel in empty space?

Light is a form of energy that makes things visible to us. It travels in straight lines without needing air or water, moving at approximately $3,00,000\,\text{km/s}$. Sunlight takes about $8\text{ minutes}$ to reach Earth.

How do luminous objects differ from non-luminous objects?

Luminous objects give out their own light (such as the Sun, stars, fireflies, electric bulbs, and candles). Non-luminous objects have no light of their own (such as the Moon, books, and mirrors) and are seen only when light from a source reflects off them into our eyes.

Why does a star act as a point source of light even though it is physically larger than the Sun?

Whether a light source acts as a point source or an extended source depends on its size compared to its distance. Stars are so far away that they appear as tiny dots, causing their light to arrive from a single point.

How do transparent, translucent, and opaque materials differ in how they transmit light and form shadows?

Transparent materials allow almost all light through to give clear views and almost no shadow. Translucent materials allow some light through, scattering it to give blurry views and faint shadows. Opaque materials block all light completely, forming dark shadows.

Identify the light path and the three types of light beams shown in this diagram.

From left to right: a ray (one single path of light), a parallel beam (rays travel side by side and never meet), a convergent beam (rays come together to meet at a point), and a divergent beam (rays spread out from a point).

What three things are required for a shadow to form?

A shadow requires a source of light, an opaque object to block the light, and a screen (such as a wall, floor, or the ground) on which the dark patch is cast.

How does changing an object's position relative to the light source and screen change the size of its shadow?

Moving the object closer to the light source makes its shadow bigger, while moving the object closer to the screen makes its shadow smaller.

In the shadow cast by an extended light source, what is the difference between the umbra and the penumbra?

The umbra is the completely dark, central part of the shadow where no light from any part of the source reaches. The penumbra is the partly dark (grey) region surrounding the umbra where light from only part of the extended source reaches.

What relative positions do the Sun, Moon, and Earth occupy during a solar eclipse, and on what lunar phase can it happen?

The Moon passes directly between the Sun and the Earth in a straight line, casting its shadow onto the Earth. A solar eclipse can happen only on a new moon day (amavasya).

Why does the Moon often appear a dull copper-red during a total lunar eclipse instead of completely disappearing?

A small amount of sunlight bends (refracts) as it passes through the Earth's atmosphere and reaches the Moon inside Earth's umbra, preferentially transmitting red wavelengths.

Why is an eclipse not observed during every new moon and full moon?

The Moon's orbit around the Earth is tilted, meaning that during most months the Moon passes slightly above or below the direct line connecting the Sun and Earth, causing its shadow to miss.

What is refraction of light, and why does it occur?

Refraction is the bending of light when it passes at an angle from one transparent medium into another. It happens because light travels at different speeds in different materials.

How are optically denser and optically rarer media defined?

An optically denser medium is one in which light travels slower. An optically rarer medium is one in which light travels faster. Optical density depends on the speed of light in the material, not on physical mass density.

Identify the labels marked in this refraction diagram for light traveling from air into glass.

The incident ray arrives in air; the normal is the dashed line perpendicular ($90^\circ$) to the surface; $i$ is the angle of incidence between the incident ray and normal; $r$ is the angle of refraction between the refracted ray and normal; and the refracted ray bends towards the normal ($r < i$).

How does the path of a light ray behave in the three cases shown in this diagram?

When entering an optically denser medium (air to glass), light slows down and bends towards the normal. When entering an optically rarer medium (glass to air), light speeds up and bends away from the normal. When striking along the normal ($i = 0^\circ$), light changes speed but does not bend at all.

Why is kerosene considered optically denser than water even though it floats on water?

Although kerosene has a lower mass density than water, light travels slower in kerosene ($\approx 2,08,000\,\text{km/s}$, $n = 1.44$) than in water ($\approx 2,25,000\,\text{km/s}$, $n = 1.33$), making kerosene optically denser.

What is the formula for refractive index ($n$), and what are its units?

The formula is $n = \frac{c}{v}$, where $c$ is the speed of light in vacuum and $v$ is the speed of light in the medium. Because it is a ratio of two speeds, refractive index has no units and is always $1$ or greater.

Why does a Pyrex glass beaker become virtually invisible when lowered into vegetable oil?

Pyrex glass and vegetable oil share nearly identical refractive indices. Because there is no difference in refractive index at the boundary, light passes straight through without bending or reflecting.

Why does an object submerged under water appear to be at a shallower depth than its real depth?

Light rays coming from the underwater object bend away from the normal as they enter the rarer air. The human brain traces these rays straight back, perceiving the virtual image of the object above its true position.

If light travels at $2,00,000\,\text{km/s}$ inside a substance, what is its refractive index ($c = 3,00,000\,\text{km/s}$)?

The refractive index is $n = \frac{c}{v} = \frac{3,00,000\,\text{km/s}}{2,00,000\,\text{km/s}} = 1.5$.

What is the difference between regular reflection and diffuse reflection?

Regular reflection occurs on smooth, shiny surfaces (like plane mirrors) where parallel incident rays remain parallel upon reflection, forming a clear image. Diffuse reflection occurs on rough surfaces where parallel rays scatter in all directions, forming no image but making the surface visible from any angle.

State the two laws of reflection illustrated in this ray diagram.

1. The angle of incidence equals the angle of reflection ($i = r$).\n2. The incident ray, the reflected ray, and the normal all lie in the same flat plane. (Both angles are measured from the normal).

What occurs when a ray of light strikes a plane mirror along the normal?

The angle of incidence is $i = 0^\circ$, so the angle of reflection is $r = 0^\circ$. The light ray bounces straight back along the exact same path it arrived on.

If an incident ray strikes a mirror surface making an angle of $30^\circ$ with the mirror itself, what is the angle of reflection?

The angle of reflection is $60^\circ$. Angles must be measured from the normal: $i = 90^\circ - 30^\circ = 60^\circ$. By the first law of reflection, $r = i = 60^\circ$.

What are the five main characteristics of an image formed by a plane mirror?

The image is virtual (cannot be caught on a screen), erect (upright), the same size as the object, located as far behind the mirror as the object is in front ($d_{\text{image}} = d_{\text{object}}$), and laterally inverted (left and right reversed).

Why is the word AMBULANCE painted mirror-wise on the front of emergency vehicles?

Because plane mirrors laterally invert images, drivers looking at the vehicle ahead in their rear-view mirrors see the inverted letters flipped back to normal, reading AMBULANCE correctly.

What minimum height must a plane mirror have for a person $150\,\text{cm}$ tall to see their entire body from head to toe?

A full-length view requires a mirror that is at least half the person's height, which is $\frac{150\,\text{cm}}{2} = 75\,\text{cm}$, hung at the proper level. Moving farther back does not change this requirement.

What formula calculates the number of images ($n$) formed by two plane mirrors placed at an angle $\theta$?

The formula is $n = \frac{360^\circ}{\theta} - 1$. For example, at an angle of $90^\circ$, the number of images formed is $n = \frac{360^\circ}{90^\circ} - 1 = 4 - 1 = 3$ images.

Why do two parallel plane mirrors ($\theta = 0^\circ$) produce an endless line of images that become progressively dimmer?

Light repeatedly bounces back and forth between the two mirrors, creating an endless series of images. Each reflection absorbs a small percentage of the light, making each subsequent image dimmer than the previous one.

How does a periscope use plane mirrors to allow an observer to see over an obstacle?

It uses two parallel plane mirrors mounted inside a tube, each fixed at an angle of $45^\circ$. Light entering the top window turns $90^\circ$ at the first mirror, travels down the tube, turns $90^\circ$ at the bottom mirror, and enters the viewer's eye as an upright image.

How did Apollo astronauts in 1969 use optical reflection to enable precise Earth-Moon distance measurements?

Astronauts placed special corner reflectors on the lunar surface. Scientists on Earth fire laser beams at these reflectors and measure the round-trip travel time of the reflected light, determining the distance to the Moon within a few centimetres.