Reflection & Refraction
State the law of reflection (angle of incidence = angle of reflection), explain how light changes speed when crossing between materials, and predict whether its direction changes; use ray diagrams for flat mirrors and describe ray paths at air-water or air-glass boundaries
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 slows down and bends towards the normal when entering water or glass from air at a slant
1 · Read
Light travels in straight rays between surfaces and reflects at a flat mirror. Centre a protractor where the ray hits, with its baseline along the mirror. Mark 90 degrees; use a ruler to draw the normal through this mark and the hit point. Now align the baseline with the normal. Read the angle to the incident (incoming) ray: the angle of incidence. The reflected (outgoing) ray's angle to the normal is the angle of reflection. The law of reflection says they are equal.
A flat mirror forms a virtual image. Draw two rays from one point on the object and reflect them using equal angles. Extend the reflected rays backward as dashed lines. Their meeting point behind the mirror is the apparent image point. The reflected rays themselves travel in front of the mirror.
A flat mirror's virtual image is upright and the same size as the object. It appears as far behind the mirror as the object is in front. Measure these distances at right angles to the mirror.
Light travels more slowly in glass or water than in air. When a ray enters either from air at a slant, it slows down and bends toward the normal. This is refraction. 'Optically denser' means light travels more slowly, not that the material weighs more.
Look from above at a straw partly under water. Light from the underwater part crosses the water's surface at a slant on its way to your eye. It changes direction there. Your eye traces the arriving rays backward in straight lines, so that part appears shifted compared with the part above water. The straw itself has not bent.
For a ray that passes from water into air at a slant, light speeds up and bends away from the normal. This rule describes the ray that crosses into air. At some larger angles, no ray crosses into the air.
An incoming ray splits the 90-degree angle between mirror and normal into two parts. At 20 degrees from the surface, it is 90 - 20 = 70 degrees from the normal. The reflected angle is also 70 degrees. Centre a protractor at the hit point with its baseline along the normal. Mark 70 degrees on the other side, in front of the mirror. Draw the reflected ray from the hit point through this mark using a ruler, with an arrow pointing away.
A ray travelling along the normal meets the boundary at 90 degrees to the surface. Its angle of incidence is 0 degrees. When it passes from water into air, it speeds up but keeps the same direction. A speed change does not always cause a bend.
Reflection gives equal angles from the normal. Crossing between air and water or glass changes light's speed. Rays at a slant bend; rays along the normal keep their direction.
The rewritten readings were written for this dataset. Linked video: How do glasses help us see? - Andrew Bastawrous and Clare Gilbert, TED-Ed. Publisher license terms have not been verified.
2 · Watch
Take it off screen
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.