You see your face in a mirror because light from your face bounces off the mirror and into your eyes; light turning back like this is called reflection.
When light reflects, the incoming and outgoing rays make equal angles with the line drawn perpendicular to the mirror (the normal), which is the law of reflection.
When light enters another material, such as going from air into water at a slant, its direction bends at the boundary; this is refraction, and because light slows down in water, it bends toward the normal.
Change the angle of incidence from 5° to 80° and compare the two pictures: the angle of reflection follows the angle of incidence exactly, while the angle of refraction always stays smaller.
Light Bouncing Off a Mirror: Reflection
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Reflection: measured from the normal, the angles of incidence and reflection are always equal
40°
💡 The Law of Reflection
①The angle of incidence always equals the angle of reflection.
②The angles are measured not from the mirror's surface but from the normal, the line perpendicular to the mirror at the point where the light hits.
③The incident ray, the reflected ray and the normal lie in one plane.
④A smooth surface reflects light neatly in one direction (regular reflection), so it shows an image like a mirror.
⑤On a rough surface like paper, each tiny patch still obeys the law of reflection, but the patches face every which way, so the light scatters in all directions (diffuse reflection); that is why paper is visible from any angle.
Light Bending into Water: Refraction
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Refraction: light going from air into water bends toward the normal
🔍 Why Refraction Happens
①Light travels at different speeds in different materials, so crossing a boundary at a slant bends its path.
②Going from air into water, light slows down and bends toward the normal; in the picture, an angle of incidence of 40° gives a refraction angle of about 29°.
③Coming out of water into air, it bends the other way, away from the normal.
④Light that meets the boundary straight on (angle of incidence 0°) goes straight through without bending.
⑤Water looks shallower than it is because light from the bottom bends as it leaves the surface and seems to come from higher up.
Rules of Reflection and Refraction
Law of reflection
angle of incidence = angle of reflection
Both angles are measured from the normal.
Rule of refraction
light from a faster to a slower material: bends toward the normal
From air into water or glass, the angle of refraction is smaller than the angle of incidence.
🌈 Refraction in Everyday Life
①A straw in a glass of water looks bent at the surface.
②A rainbow forms when sunlight refracts entering and leaving raindrops, and each color bends by a different amount, so they separate.
③A mirage that looks like a puddle on hot asphalt comes from light bending through layers of air at different temperatures.
④Glasses and magnifying lenses use refraction to control where light comes together or spreads out.
Exam Key Points
Summary
reflection: angle of incidence = angle of reflection · refraction: light bends when the material changes
The law of reflection holds on any surface, and refraction happens at a boundary where the speed of light changes.
🎯 Exam Points
①Law of reflection: angle of incidence = angle of reflection (measured from the normal).
②Regular reflection happens on smooth surfaces and diffuse reflection on rough ones; both obey the law of reflection.
③Air → water: angle of refraction < angle of incidence (bends toward the normal).
④Water → air: angle of refraction > angle of incidence (bends away from the normal).
⑤Light entering straight on does not bend.
Examples and Unit Test
Example 1
Light strikes a mirror at an angle of incidence of 40°. What is the angle of reflection?
1
By the law of reflection, the angles of incidence and reflection are always equal, both measured from the normal.
2
The angle of incidence is 40°, so the angle of reflection is also 40°.
▸ 40°
If a question gives the angle between the ray and the mirror's surface, subtract it from 90° to get the angle from the normal first.
Example 2
When light passes at a slant from air into water, is the angle of refraction larger or smaller than the angle of incidence? Why?
1
Light travels more slowly in water than in air, so a slanting ray bends toward the normal at the boundary.
2
Bending toward the normal makes the angle of refraction smaller than the angle of incidence.
▸ Smaller (it bends toward the normal)
Entering a material where light is slower, it bends toward the normal; leaving into one where it is faster, it bends away.
Unit-test style
Which statement about reflection and refraction of light is correct?
①When light reflects, the angle of incidence is always larger than the angle of reflection
②In reflection, the angle of incidence and the angle of reflection are always equal
③When light goes from air into water, the angle of refraction is larger than the angle of incidence
④Angles are measured from the surface of the mirror
⑤Light does not bend even when it enters a different material
▸ ② In reflection, the angle of incidence and the angle of reflection are always equal
1
By the law of reflection, the angles of incidence and reflection are always equal, so ② is correct.
2
In reflection the two angles are equal (①), from air into water the angle of refraction is smaller (③), angles are measured from the normal (④), and light bends when it enters another material at a slant (⑤).