Reflection and Refraction

Reflection

Reflection occurs when light traveling through one material bounces off a different material. The reflected light still travels in a straight line, only in a different direction. The light is reflected at the same angle that it hits the surface. The angle of incidence is equal to the angle of reflection. The angle of incidence is the angle between the incoming light and a line perpendicular to the surface called the normal. The angle of reflection is the angle between the reflected light and the normal. The symbol Ɵ means “angle'' and arrows represent rays of light.

Light reflecting off a smooth surface, where all of the light is reflected in the same direction, is called specular reflection. Along a smooth surface, the normal always points the same way, so all of the light is reflected in the same direction (A on the picture below) and the image that is reflected looks the same as the original image.

The normal at different spots along the rough surface points in different directions, which causes the reflected light to go in different directions. This is called diffuse reflection.  The arrows show in which direction the reflected image will appear when light reflects off a rough surface (B). 

The headlights of a car shine onto the road at night. If the road is dry, the light is diffusely reflected (A), since the pavement is very rough. If the road is wet, the water makes the road surface smoother. There is more specular reflection of the light from the car’s headlights (B). This causes glare (light reflected off the surface like a mirror) that makes it hard for drivers to see.

Refraction

When light traveling through one material reaches a second material, some of the light will be reflected, and some of the light will enter the second material. At the point at which the light enters the second material, the light will bend and travel in a different direction than the incident light. This is called refraction. Refraction happens because the speed of light is different in different materials (though always less than the speed of light in a vacuum).

Think about pushing a shopping cart along cement, and then reaching grass, like in the picture below. It’s harder to push the cart in the grass, so each wheel slows down when it reaches the grass. The wheels on the pavement are still moving faster, so it causes the cart to change directions (in this case, turn to the right).

Index of Refraction

Materials have a property called the index of refraction, which is symbolized using the letter n. The index of refraction of a material is equal to the speed of light in a vacuum, divided by the speed of light in the material. The higher the index of refraction, the slower light travels in that medium. If light is traveling in one material and then refracts in a second material, it will bend towards the normal if the index of refraction of the second material, n₂, is greater than the index of refraction of the first material, n₁ (the light travels slower in the second material) (n₁ < n₂) (A). If the second material has a lower index of refraction, the light will bend away from the normal as it travels faster in the second material (n₁ > n₂) (B). Unlike reflection, the angle of incidence is not equal to the angle of refraction.

The angle of incidence and angle of refraction are mathematically related to the index of refraction of each material through the law of refraction, also called Snell’s Law.

Converging and Diverging Light

A lens is an optical device made of plastic or glass. When light passes through a lens, it can be refracted in predictable ways depending on the shape of the surfaces of the lens. The surface of a lens may be convex (curved outward) or concave (curved inward). When parallel beams of light strike a lens that is convex on both sides (double convex lens) the light is refracted inwards and is said to be converging (A). The beams of light cross at a point called the focal point which is behind the lens (to the right of the lens). When light strikes a lens that is concave on both sides (biconcave lens), the light is refracted outwards and is said to be diverging (B). In this case, the focal point is actually in front of (to the left of) the lens.

Light through a Prism

Light entering a plastic or glass prism (usually a triangular prism), refracts first as it enters the prism and again as it exits the prism. When white light passes through a prism, it is refracted into all of its colours. If you project this light onto a white surface, you see what looks like a rainbow. All of the colours that make up white light are separated at different angles. This is because each individual colour refracts by a different amount through the prism.