You are taking a selfie outside on a sunny day. The bright light from the Sun lets your camera capture your face clearly. But why can you see your face, the colors of your clothes, and the reflection in your sunglasses? All of this happens because of how light behaves when it hits different materials. Understanding this behavior is not only cool for selfies, but it is also the science behind cameras, glasses, fiber-optic internet, and even rainbows.
Light is a form of energy called electromagnetic radiation. It travels in waves, but it is very different from waves on water or sound waves in air.
This is why we can see the Sun and stars even though space is almost completely empty. Light does not need particles of matter to carry it. That means light cannot be a mechanical wave like sound or water; instead, it is an electromagnetic wave.
Light travels extremely fast. In empty space, it moves at about \(3.0 \times 10^8 \textrm{ m/s}\), which is 300,000,000 meters per second. That is fast enough to go around Earth more than 7 times in one second!
The light we see with our eyes is called visible light, and it is only a small part of the entire electromagnetic spectrum, which also includes radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.
As shown in [Figure 1], different types of electromagnetic radiation are arranged by frequency and wavelength, with visible light in the middle.
Light as a wave has two important properties:
Frequency and wavelength are related. If a wave has a high frequency, it has a short wavelength; if it has a low frequency, it has a long wavelength. For light, the color we see depends on its frequency (or wavelength):
Inside visible light, the colors go in order: red, orange, yellow, green, blue, indigo, violet. You can remember this with “ROY G. BIV.”
In a uniform material like air, light travels in straight lines. We often draw light using rays—straight arrows that show the direction in which the light is moving. This is called a ray model of light.
For example:
Using rays helps us predict where shadows will form, how mirrors work, and how lenses focus light.
When light shines on an object, three main things can happen:
Most real objects do a combination of these, but one effect is usually strongest.
Reflection is why you can see yourself in a mirror. When light hits a smooth, shiny surface, it bounces off in a regular way.
We can summarize this with the idea: angle in = angle out, or more formally, the angle of incidence equals the angle of reflection.
Dull surfaces, like paper, also reflect light, but in many scattered directions. This is called diffuse reflection, and it lets us see most everyday objects.
When light is absorbed, its energy is transferred to the material. Often, this energy turns into heat.
The color of an object that you see depends on which colors (frequencies) of light it reflects and which it absorbs:
This means the material and the frequency (color) of the light both matter.
Some materials let light pass through them. This is called transmission. There are three main types of materials based on how they transmit light:
For example, a glass window is transparent to visible light but may block some ultraviolet light. Sunglasses are designed to transmit some light while blocking harmful UV.
As shown in [Figure 2], you can compare reflection, absorption, and transmission for the same light ray hitting different types of materials.
Up to now, we have said light travels in straight lines. That stays true inside one material, like air or water. But when light passes from one transparent material into another (like air into water), something interesting happens: its path bends. This bending of light is called refraction.
You can see refraction in everyday life:
Why does this happen? Light travels at different speeds in different materials. It moves fastest in a vacuum, a bit slower in air, slower in water, and even slower in glass. When light changes speed as it enters a new material at an angle, it changes direction.
Key ideas for refraction:
Refraction helps explain many useful devices, like lenses and prisms.
Lenses are carefully shaped pieces of transparent material (usually glass or plastic) that use refraction to bend light in specific ways. Lenses are used in eyeglasses, microscopes, telescopes, cameras, and magnifying glasses.
There are two main types of lenses:
As [Figure 3] illustrates, convex and concave lenses bend light in opposite ways and are used for different purposes.
Convex lenses bend light rays inward, toward a point called the focal point. When parallel light rays (like rays from a distant object) enter a convex lens:
Uses of convex lenses:
Concave lenses bend light rays outward, as if they are spreading from a point. When parallel light rays enter a concave lens, they diverge (spread apart).
Uses of concave lenses:
Both convex and concave lenses rely on the change in speed of light as it passes between air and glass, and on how much the path of light bends at each surface.
A prism is a transparent object (often a triangular glass block) that uses refraction to separate white light into a spectrum of colors.
When white light enters a prism, it is made of many different frequencies (colors). Each frequency bends by a slightly different amount because the speed of light in glass depends on its frequency. Higher-frequency light (like violet) slows down a bit more and bends more than lower-frequency light (like red) in the same material.
As light enters and then exits the prism, the colors spread out into a rainbow: red, orange, yellow, green, blue, indigo, violet. This is called dispersion.
Rainbows in the sky form in a somewhat similar way: sunlight enters raindrops, refracts, reflects inside the drops, and then refracts again, spreading into colors.
To understand some behaviors of light, especially color, brightness, and bending in prisms, it helps to think of light as a wave.
This wave model of light is especially useful when explaining:
Even though light behaves like a wave in many ways, it is not a matter wave like sound or water waves:
This is why you cannot hear sounds in space (no air to vibrate), but you can see light from stars.
The way light is reflected, absorbed, transmitted, and refracted is used in many technologies that affect your everyday life.
Eyeglasses use lenses to correct vision:
Smartphone cameras use convex lenses to focus light onto a small sensor. The lens position changes to keep different objects in focus, similar to how your eye’s lens changes shape.
Screens (phone, tablet, TV) create colors by combining tiny red, green, and blue light sources at different intensities. Your eyes and brain mix these to see millions of colors.
Fiber-optic cables use light to carry information for the internet and phone calls. Inside these cables, light bounces along the glass core, being reflected and refracted in a controlled way so it stays inside the cable over very long distances.
This allows huge amounts of data—videos, messages, game updates—to travel quickly across the world.
Solar panels absorb light from the Sun and turn it into electrical energy. The materials in the panel are chosen to absorb certain frequencies of light very well, making them more efficient.
Lasers are beams of very pure, single-frequency (single-color) light. Because the waves are all the same frequency and line up in step, lasers can carry energy very precisely.
1. Refraction with a Straw
This shows refraction at the boundary between air and water.
2. Making a Small Rainbow
This demonstrates how different frequencies of light bend by different amounts (dispersion).
3. Transparent, Translucent, Opaque Test
1. Light is electromagnetic radiation that can travel through empty space. It is not a mechanical wave like sound or water waves.
2. Light travels in straight lines within a single medium and can be represented by rays, but it bends (refracts) at boundaries between different transparent materials like air, water, and glass.
3. When light hits an object, it can be reflected, absorbed, or transmitted. The material and the frequency (color) of the light determine what happens.
4. Color is related to the frequency (or wavelength) of light. Objects appear certain colors based on which frequencies they reflect and which they absorb.
5. Lenses use refraction to focus or spread light. Convex lenses bring light rays together and are used in magnifying glasses, cameras, and some eyeglasses. Concave lenses spread light rays apart and are used in other types of eyeglasses and devices.
6. Prisms bend different frequencies of light by different amounts, spreading white light into a rainbow. This shows that light of different colors has different frequencies and behaves slightly differently in materials.
7. Understanding light’s behavior is essential for many modern technologies, including eyeglasses, cameras, fiber-optic communication, solar panels, and lasers.
