Reflection and Ray Diagrams
- Reflection can be either specular or diffuse.
→ Diffuse refers to rough surfaces that scatter light in all directions.
→ Specular: Smooth surfaces; reflection can be easily viewed.
The law of reflection states the angle of incidence must equal the angle of reflection.
→ Angle i = Angle between incident ray & normal.
→ Angle r = Angle between reflected ray & normal.
- Normal line = Imaginary line 90° to surface where reflection occurs.
- Image = A point in space a virtual image can be considered to have light rays actually passing through, the light rays appear to meet but actually don't.
- Plane mirrors generate virtual images, or images that don't exist.
- Virtual rays aren't real rays of light. Light doesn't actually emerge from a virtual image, but an observer doesn't know this just by looking; our brains assume that light travels in straight lines and is coming from behind the mirror.
Examples:
- When light rays bouncing off an object onto a mirror diverge, they form virtual images, giving the impression that the light is coming from a completely different place.
Refraction
- Light bends as it enters a glass block due to the slower travel speed of light in glass. This leads to a reduction in the wavelength and a change in the direction of the light.
- Light bends toward the normal line when it enters denser materials.
- Light bends away from the normal line when it enters less dense materials.
- Refraction describes the change of wave speed of a wave when entering or leaving a material, causing it to bend. Dense material = slower light speed.
- The deeper the material, the more light enters it and the more it bends away from the normal line.
Wave Light
All the rainbow colours of the spectrum combine to form white light.
- Different colours—different wavelengths of light. These wavelengths travel in glass at different speeds, with some refracting more than others.
- Red light, having the longest wavelength, changes its speed and refracts the least.
- Violet light, having the shortest wavelength, alters its speed and refracts most.
- Changes in speed mean white light disperses into colours of the spectrum as it travels through a prism.
Experiment
Conduct an experiment on glass blocks using a ray box, protractor, and ruler - in glass blocks
Refractive Index
- The difference in speed that light travels in materials causes refraction.
- Every material has a refractive index. The higher the refractive index, the slower the speed of light in the material.
- The larger the difference in refractive index between two materials, the smaller the angle of refraction.
- Refractive index can be calculated using:
- The refractive index can also be calculated using this formula:
- The speed of light in a vacuum is 3x10⁸ m/s¹.
- Air has a refractive index of 1.00, so the speed of light in air is very similar to the speed of light in a vacuum. Any refractive index is always > 1.
E.g., 1: Calculate the speed of light in diamond (RI = 2.42).
Light travels fastest in gases.
Refractive Indexes of Substances (2 d.p.)
| Material | Speed of light in material (m/s) | Refractive Index |
|---|---|---|
| Air | 3.0 x10⁸ | 1 |
| Glass | 2.0 x10⁸ | 1.58 |
| Water | 2.3 x10⁸ | 1.36 |
| Diamond | 1.2 x 10⁸ | 2.50 |
| Turpentine | 2.2 x 10⁸ | 1.50 |
Finding the Refractive Index of a BLOCK
APPARATUS
- Power supply + leads
- A ray box with a thin slit
- Rectangular glass block
- Protractor
- Plain A4 paper
- Sharp pencil
METHOD
1) Place the rectangular block in the middle of the paper and draw around it.
2) Remove the block and draw a normal line, as shown below:
3) Measure an angle at a distance of 10° from the normal and draw a line along it.
4) Replace the block in its original position and direct a ray of light along a 10° line into the block.
5) Mark using 'x's where the light emerges and join.
6) Join up the lines as follows to get the refraction:
7) Now measure the angle of refraction.
Now repeat more angles.
Next, plot a graph with sin i on the x-axis.
sin(r) (y-axis)
The gradient will be the refractive index.
Critical Angles
- At an incident angle called the critical angle, the light is refracted at 90° and travels along the edge of the material.
- The critical angle for light leaving a glass block into air = 42°
- The critical angle for light leaving water into air = 49°
- The critical angle for light leaving diamond into air = 24°
- The critical angle for light leaving cubic zirconia into air = 28°
- The slower light travels in a material, the higher its RI and the smaller its critical angle at an air boundary.
- We can calculate the critical angle (c) using: 
E.g., Calculate the critical angle of a substance where RI = 2.
METHOD TO INVESTIGATE C:
1) Place the perspex block on A4 paper and draw around it.
2) Draw normal lines to the bottom of the block.
3) Shine light from ray box onto block so angle i = 10°.
4) Draw the emerging ray, measure it, and record the angle r.
5) Repeat for other angles.
Total Internal Reflection (TIR)
- When a substance's angle i exceeds its angle c, the light cannot refract and returns entirely to the material through reflection.
OPTICAL FIBRES:
- An optical fibre, a long, thin glass rod with cladding surrounding it, uses TIR to transfer information by light, even when bent.
- Extensively used in medicine and communications.
- A pulse of light enters fibres at one end.
- Sharp right-angle bends at the end ensure light reflects and is refracted until it emerges at the other end.
- It functions by bouncing waves off a narrow core slab.
- Two right-angle triangle prisms can be used to make a periscope. On the back face of the prism, there is TIR.
