6.1 Refraction of Light
Refraction of light is an optical phenomenon that occurs when light travels between mediums of different optical densities, causing everyday effects such as a pencil appearing bent in water or a swimming pool appearing shallower than its actual depth.
Learning Objectives
- Explain the concept of refraction of light and the cause of light bending.
- Compare the direction of bending and velocity of light when travelling between mediums of different optical densities.
- Define refractive index, \(n\), as the ratio of the speed of light in vacuum to the speed of light in a medium.
- State and apply Snell's Law.
- Describe experiments to determine the refractive index of a glass block and water using the non-parallax method.
- Define real depth, \(H\), and apparent depth, \(h\), and solve problems using \(n = \frac{H}{h}\).
Refraction Ray Paths, Snell's Law, and Depth Perception
Compare how light bends when entering denser and less dense mediums, and see how real depth and apparent depth are related.
Air to Glass: Bends Towards Normal
Glass to Air: Bends Away From Normal
Short Explanation
What is Refraction?
Refraction of light is the bending of light rays that occurs due to a change in the velocity of light when travelling through mediums of different optical densities.
Optical Density vs Mass Density
Optical density is not the same as physical/mass density. Oil has a lower mass density than water, but oil has a higher optical density than water.
Refractive Index
Refractive index, \(n\), determines the degree to which light bends when travelling from vacuum into a medium. \(n = \dfrac{c}{v}\)
Snell's Law
Snell's Law connects the refractive indices and angles of light across two media. \(n_1 \sin \theta_1 = n_2 \sin \theta_2\)
| Medium | Refractive Index, \(n\) |
|---|---|
| Vacuum and air | 1.00 |
| Olive oil | 1.46 |
| Perspex | 1.50 |
| Glass (crown) | 1.52 |
| Glass (flint) | 1.66 |
| Diamond | 2.42 |
Try to Answer First
Answer in your mind, then press “Check Answer”.
A light ray travels from air into a diamond with a refractive index of 2.42 at an angle of incidence of 30°. Calculate the angle of refraction in the diamond.
Check Answer
The speed of light in vacuum is \(3.0 \times 10^8\ \mathrm{m\,s^{-1}}\). If the refractive index of a type of plastic is 1.50, what is the speed of light in this plastic?
Check Answer
A swimming pool has an actual depth of 2.4 m. Given that the refractive index of water is 1.33, what is the apparent depth of the pool when viewed directly from above?
Check Answer
Common Mistakes
- Measuring angles from the surface instead of the normal line.
- Confusing mass density with optical density. Cooking oil floats on water, but oil has a higher optical density than water.
- Inverting the real depth and apparent depth ratio. The correct formula is \(n = \frac{H}{h}\), not \(n = \frac{h}{H}\).
- Misapplying the ratio \(\frac{\sin i}{\sin r}\) by assuming \(i\) is always the angle at the top, rather than the angle of incidence in air.
Concept Misunderstandings
Light bends because the boundary physically pushes or deflects the ray.
Refraction occurs solely because the wave speed of light changes as it moves between media of different optical densities.
The refractive index can be less than 1.
The speed of light in vacuum is the maximum possible speed, so \(n = \frac{c}{v}\) is always greater than or equal to 1.
A diver looking at someone on land also sees them closer to the surface.
When light travels from air into water, it bends towards the normal. The diver sees the virtual image of the instructor located further away from the pool surface.
Summary
- Refraction is the change in direction of light caused by a change in velocity across media of different optical densities.
- Less dense to denser: bends towards normal, speed decreases, \(i > r\).
- Denser to less dense: bends away from normal, speed increases, \(i < r\).
- Refractive index: \(n = \frac{c}{v}\), where \(c = 3.0 \times 10^8\ \mathrm{m\,s^{-1}}\).
- Snell's Law: \(n_1 \sin \theta_1 = n_2 \sin \theta_2\).
- Real depth and apparent depth: \(n = \frac{H}{h}\).
Short Activity
Objective: Identify refraction, refractive index, Snell's Law and depth perception.
A. Objective Quiz
1 What causes the refraction of light when it passes from air into a glass block?
2 A ray of light travels from water into crown glass. Given that crown glass is optically denser than water, how does the light ray behave upon entering the glass?
B. Fill in the Blanks
3 The ratio of the speed of light in vacuum to the speed of light in a medium is known as the index.
4 When light travels from air into a liquid, the formula relating real depth \(H\) and apparent depth \(h\) to refractive index \(n\) is \(n =\) .
C. Matching / Drag and Drop
Drag each term to the correct description. If using a phone, tap the answer first, then tap the matching box.
