How light travels
Light comes from a source, like the Sun, a bulb or a fire. Most things we see are not sources. We see them because light bounces off them into our eyes.
Light travels in straight lines. We draw it as a line with an arrow, called a ray.
Light is a wave, but it is different from sound and water waves:
- Sound needs a material (air, water, metal) to travel. Light does not. It crosses empty space from the Sun to Earth.
- Light is much faster. In empty space it goes about 3 × 10⁸ m/s (300 000 km each second). Sound in air goes only about 340 m/s.
That is why you see lightning before you hear thunder.
Reflected, absorbed or transmitted
When light meets an object, three things can happen:
- Reflected: the light bounces back. Mirrors and white paper reflect a lot.
- Absorbed: the light is taken in by the material. Its energy warms the material. Black things absorb a lot.
- Transmitted: the light passes through. Clear glass and water transmit a lot.
Most materials do a mix. Clear glass transmits about 90%, but you can still see a faint reflection in a window.
Transparent, translucent, opaque
- Transparent: light goes straight through; you see clearly (clear glass).
- Translucent: light goes through but is scattered; you see a blur (frosted glass, tracing paper).
- Opaque: no light goes through; it makes a shadow (wood, metal).
Reflection and mirrors
Draw a line at right angles (90°) to the surface where the ray hits. This line is the normal.
- Angle of incidence (i): between the incoming ray and the normal.
- Angle of reflection (r): between the reflected ray and the normal.
Law of reflection: i = r.
A smooth surface (mirror, still water) gives specular reflection: all rays bounce the same way, so you see an image. A rough surface (paper, a wall) gives diffuse reflection: rays scatter in many directions, so you see the surface but no image.
An image in a plane mirror is the same size, upright, as far behind the mirror as the object is in front, and left-right reversed.
Refraction, lenses, the eye and the pinhole camera
Light moves slower in glass and water than in air. When it enters at an angle, it bends. This is refraction.
- Into a slower material (air → glass): bends towards the normal.
- Into a faster material (glass → air): bends away from the normal.
- Along the normal (angle 0°): it slows but does not bend.
A convex lens (fat in the middle) bends rays together to a point, the focus. A concave lens (thin in the middle) spreads them out.
A pinhole camera is a box with a tiny hole. Rays travel straight through the hole and make an upside-down image on the back. Your eye works in a similar way, but it has a lens to make the image sharp. The image falls on the retina, and the brain turns it the right way up.
Colour and prisms
White light is a mix of colours. A glass prism bends each colour a different amount, so they spread out into a spectrum: red, orange, yellow, green, blue, indigo, violet. Red bends least; violet bends most. This spreading is called dispersion. A rainbow is made the same way by raindrops.
- An object's colour is the colour it reflects. A green leaf reflects green and absorbs the other colours.
- White objects reflect all colours. Black objects absorb all colours.
- A filter transmits only its own colour. A red filter lets red through and absorbs the rest.
A red shirt under pure blue light looks black: there is no red light for it to reflect.
Key formulas and definitions
- Law of reflection: angle of incidence = angle of reflection (i = r)
- Speed of light in empty space ≈ 3 × 10⁸ m/s
- distance = speed × time
- Reflected % + absorbed % + transmitted % = 100%
Worked examples
1. A ray hits a mirror at 35° to the normal. What is the angle of reflection? What is the angle between the incoming and reflected rays?
Step 1: Law of reflection: r = i = 35°. Step 2: Angle between the two rays = i + r = 35° + 35° = 70°.
2. A ray hits a mirror at 20° to the mirror surface (not the normal). Find the angle of reflection.
Step 1: The normal is at 90° to the surface. Step 2: Angle to the normal i = 90° − 20° = 70°. Step 3: r = i = 70°.
3. Light from the Sun takes about 500 s to reach Earth. Light travels at 3 × 10⁸ m/s. How far away is the Sun?
Step 1: distance = speed × time. Step 2: = 3 × 10⁸ × 500 = 1.5 × 10¹¹ m. Step 3: That is 150 million km.
4. A window transmits 88% of light and absorbs 4%. What percentage does it reflect?
Step 1: The three parts add to 100%. Step 2: Reflected = 100 − 88 − 4 = 8%.
5. Why does a green T-shirt look black under red light?
Step 1: The shirt only reflects green light. Step 2: Red light has no green in it. Step 3: The shirt absorbs the red, reflects nothing, and looks black.
Common mistakes
- Measuring the angle from the mirror surface instead of from the normal. Always measure from the normal.
- Thinking we see objects because light comes out of our eyes. Light enters our eyes from the object.
- Thinking refraction always bends light towards the normal. It bends away when light goes into a faster material, like glass to air.
- Thinking a prism adds colours to light. The colours were already in white light; the prism only separates them.