REFRACTION: the change in direction of a wave when it passes from one medium to another.
Caused by a CHANGE IN WAVE SPEED at the boundary.
When waves move from a FAST medium to a SLOW medium โ bend TOWARDS the normal.
When waves move from a SLOW medium to a FAST medium โ bend AWAY from the normal.
FOR LIGHT:
Light slows down when entering glass or water from air.
Bends TOWARDS the normal when entering glass (air โ glass).
Bends AWAY from the normal when leaving glass (glass โ air).
WAVELENGTH changes during refraction โ frequency stays constant.
ฮป = v/f โ if v decreases and f stays constant, ฮป must decrease too.
Ray Diagrams for Refraction
Drawing refraction ray diagrams:
1. Draw the boundary between two media (horizontal line).
2. Draw the NORMAL โ a dashed line perpendicular to the boundary at the point of incidence.
3. Draw the INCIDENT RAY approaching the boundary.
4. Measure the ANGLE OF INCIDENCE (i) โ between incident ray and normal.
5. Draw the REFRACTED RAY on the other side of the boundary.
6. Measure the ANGLE OF REFRACTION (r) โ between refracted ray and normal.
RULE: when going from air into glass (slower medium):
Angle of refraction < angle of incidence (bends towards normal).
If incident ray is along the normal (0ยฐ): no bending occurs.
At 0ยฐ (along the normal): no refraction โ wave passes straight through.
Absorption, Transmission and Reflection
When EM waves reach a boundary, several things can happen:
TRANSMISSION: wave passes through the medium.
ABSORPTION: wave energy transferred to the medium (heats it up or causes other effects).
REFLECTION: wave bounces back from the boundary.
REFRACTION: wave passes through but changes direction due to speed change.
Different materials affect different wavelengths differently:
Glass: transparent to visible light, opaque to ultraviolet and infrared.
Sunscreen: absorbs UV, transmits visible.
X-ray: transmitted by soft tissue, absorbed by bone.
Microwaves: absorbed by water molecules in food (hence microwave cooking).
This wavelength-dependent behaviour is why we have different detectors and uses for different parts of the EM spectrum.
โ ๏ธ Common Mistake
When light enters glass from air, it SLOWS DOWN and bends TOWARDS the normal (angle of refraction < angle of incidence). When it leaves glass into air, it SPEEDS UP and bends AWAY from the normal. Always identify which medium is faster/slower.
๐ Key Note
Refraction: change in direction due to change in speed at boundary. Air โ glass (slower): bends towards normal. Glass โ air (faster): bends away. Frequency constant; wavelength changes. Ray diagrams: incident ray, normal, refracted ray, angles of incidence and refraction.
๐ฏ Matching Activity โ Refraction Concepts
Match each refraction scenario to the correct description. โ drag the symbols on the right to match the component names on the left.
Air โ glass
Drop here
Glass โ air
Drop here
Along the normal (0ยฐ)
Drop here
Frequency during refraction
Drop here
Wave speeds up โ bends away from the normal โ angle of refraction larger
No refraction โ wave passes straight through boundary unchanged
Stays constant โ wavelength changes as speed changes
Wave slows down โ bends towards the normal โ angle of refraction smaller
๐งช Required Practical
๐ฌ RP20 (Physics) โ Investigate refraction of light through glass or perspex blocks. Measure angles of incidence and refraction. Plot i vs r to find relationship.
Know the method, variables, equipment and how to analyse results.
๐ฏ Test Yourself
Question 1 of 2
1. A ray of light passes from air into a glass block. The angle of incidence is 40ยฐ. What happens to the ray?
2. When light is refracted into a slower medium, which property changes?
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