Reflection & Refraction
State the law of reflection (angle of incidence = angle of reflection) and explain refraction as the change in speed and direction when light crosses a boundary between two media; apply ray diagrams for plane mirrors and refracting surfaces
What a learner can do afterwards
- States the law of reflection and applies it to draw a reflected ray correctly
- Draws a ray diagram for a plane mirror showing a virtual image
- Explains why a pencil looks bent in a glass of water using refraction
- Explains qualitatively why light bends towards the normal when entering a denser medium
The lesson
When light hits a flat mirror, it bounces off in an exact way. Draw a normal: a line at a right angle (90 degrees) to the mirror at the point where the ray hits. The angle between the incoming ray and the normal is the angle of incidence. The angle between the bounced ray and the normal is the angle of reflection. These two angles are always equal.
A flat mirror makes a virtual image. To find it on a ray diagram, extend the reflected rays backward, behind the mirror, until they meet. That meeting point is where the image seems to be, even though no real light is actually there. The virtual image is the same size as the object and sits the same distance behind the mirror as the object sits in front.
Refraction happens when light crosses the boundary between two different materials, like air and water. The new material changes how fast light travels, so the ray bends at the boundary. Going into a denser material, like water or glass, slows light down and bends the ray toward the normal. Coming back out into a less dense material speeds light up and bends the ray away from the normal.
Put a straw in a glass of water and look at it from the side. Light from the part of the straw above water travels straight to your eye. Light from the part below water bends as it leaves the water and crosses into air. Your eye traces that bent light back in a straight line, so the underwater part looks shifted. That is why the straw looks bent right at the surface.
Quick rule: light bends toward the normal when it slows down entering a denser material, and away from the normal when it speeds up leaving one. Same rule, both directions.
Reflection bounces light at equal angles from the normal; refraction bends light because its speed changes when it crosses into a new material.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.