Light and Sound at Boundaries

Three things a wave can do at a boundary

A boundary is simply the place where one material meets another: air meeting glass, water meeting air, air meeting a brick wall. When a wave, light or sound, reaches such a boundary, exactly three things can happen to it, and most questions here are really asking which. It can be reflected, bouncing back into the material it came from. It can be refracted, passing into the new material but bending as it does. Or it can be absorbed, soaked up so that its energy is taken into the material. Usually more than one happens at once. Light hitting a window is partly reflected, so you see a faint image of yourself, and partly refracted, passing through and bending. The part students find puzzling is why a refracted wave bends, so here is the one idea to hold on to. A wave bends only because it changes SPEED when it crosses the boundary. In a material where it travels slower, it bends toward the line drawn at right angles to the surface, which is called the normal. In a material where it travels faster, it bends away from that line. No change of speed, no bending. And none of this is only about light. Sound reflects too, which is what an echo is, and sound also changes speed and refracts when it passes between materials. Carry one question through everything that follows: at this boundary, is the wave reflected, refracted because it changed speed, or absorbed?

Words for boundaries

Five terms, each defined by what it is. They are the language for what happens where two materials meet.

Tap what happens at a boundary

Tap the TWO statements that are true of a wave meeting a boundary between two materials.

• Some of the wave can bounce back into the material it came from
• A wave that crosses into the new material can bend if it changes speed
• A wave always passes straight through with no change at all
• Only light does this, and sound is never affected by boundaries

Why the wave bends

A ray of light passes from air into a glass block and bends. What actually causes the bending?

• The light changes speed as it crosses into the glass, and that change of speed is what bends its path toward the normal
• The solid glass physically shoves the ray sideways
• The colour of the glass pulls the ray over
• The ray simply bounces off the surface of the glass

Reflected against refracted

Both happen at a boundary, and telling them apart is most of the topic.

Match each behaviour to an example

Two things true of refraction

Select the TWO statements that are true about refraction.

• It happens because the wave changes speed as it crosses the boundary
• A wave that slows down bends toward the normal
• It means the wave bounces straight back the way it came
• It happens to light but never to sound

Answering a boundary question

A method that works on any question about a wave meeting a boundary. Name the boundary and the two materials. Say what is on each side, because the behaviour depends on the pair. Decide which of the three happens. Reflected, refracted, or absorbed. Often more than one, so say so. For refraction, always give the reason. The wave bends because it changes speed. Slowing down bends it toward the normal; speeding up bends it away. An answer that says a wave bends without saying it changed speed is missing the mark. For reflection, use the angle rule. The angle the wave arrives at equals the angle it leaves at, both measured from the normal. Remember sound. Everything here applies to sound as well as light. An echo is reflected sound. [Higher] Sound can also pass through solids, and the ear works by passing sound vibrations along tiny bones into the inner ear. Two habits cost marks. The first is saying light bends because the glass is in the way; it bends because of the change of speed. The second is treating this as a light-only topic; sound reflects and refracts too.

Order what happens to a ray at a boundary

A ray of light in air meets a glass block. Put the stages in order.

• A ray of light travels through the air toward a glass block
• It reaches the boundary at the surface of the glass
• Some of the light reflects back into the air
• The rest crosses into the glass
• As it crosses, it slows down and bends toward the normal

Build the refraction rule

This is the rule for which way a refracted wave bends. Assemble it.

Boundary quick-fire

Five questions on what waves do at boundaries. Three lives.

A straw that looks broken

Here is one everyday sight explained, and one everyday sound. Put a straw in a glass of water and look from the side. The straw appears bent or broken at the surface, even though you know it is perfectly straight. Nothing has happened to the straw. What has happened is to the light. As the light from the underwater part of the straw leaves the water and enters the air, it speeds up and changes direction, so it reaches your eye from a slightly different angle than you expect. Your brain assumes light travels in straight lines, so it draws the straw where the light seems to have come from, and the straw looks shifted. Now a sound. Shout across a valley toward a cliff and, a moment later, the shout comes back to you. The sound has travelled to the cliff, bounced off the hard rock, and returned. The delay is just the time for the round trip. Both are the ordinary physics of a boundary. In the glass, the light passed from one material into another and changed direction because it changed speed. At the cliff, the sound met a surface it could not pass through and came back instead. One wave passing through and bending, one wave bouncing back, and the straw and the echo are the everyday faces of the two.

Complete the boundary facts

When a wave bounces back off a boundary, that is _____. When a wave passes into a new material and bends because it changes speed, that is _____. When a wave is soaked up and its energy taken in, that is _____. Angles at a boundary are always measured from a line called the _____.

Three boundary answers to sharpen

Three student answers about waves at boundaries. In each case the reasoning is what needs work.

• A student writes that light bends in glass because the solid glass physically pushes the ray sideways. What is the real reason?
• A student uses reflection and refraction as if they mean the same thing. How would you sharpen it?
• A student says that only light reflects and refracts, and sound does neither. How would you correct it?

Explain what happens to waves at boundaries

A friend knows that light can bend but does not understand what happens to waves at a boundary between two materials. Explain it.

• Name the three things that can happen to a wave when it reaches a boundary
• Explain what reflection is, and the rule about the angles
• Explain what refraction is, and why the wave bends
• Explain which way a wave bends when it slows down, and which way when it speeds up
• Finish by giving one example for light and one for sound, showing both behave this way