3.2Waves
Light — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
30 min read 47 parts Lesson 8 of 35 Unit quiz
Lesson summary
3.2 Light (Part 1) — IGCSE PHYSICS notes: State and test the law of reflection and describe the image formed by a plane mirror.Normal = a line drawn at right angles to the boundary or mirror surfaceAngle of incidence (i) = the angle between the incident ray and the normalAngle of…
- Angle of reflection (r)
- Rough and smooth surfaces
- Rotating a mirror
- Why virtual
- Virtual image
- A reflected ray has an arrow pointing away from the boundary
Part 1 of 8Ray diagram terms
Normal
Normal a line drawn at right angles to the boundary or mirror surface
Angle of incidence (i)
Angle of incidence (i) the angle between the incident ray and the normal
Angle of reflection (r)
Angle of reflection (r) the angle between the reflected ray and the normal
Arrows
- Arrows an arrow on each ray shows the direction in which the wave travels
- An incident ray has an arrow pointing towards the boundary.
- A reflected ray has an arrow pointing away from the boundary.
Part 2 of 8Law of reflection
1Statement
- Statement the angle of incidence equals the angle of reflection, i = r, with both angles in degrees
2Uses
- Uses the law is applied in mirrors, cameras, optical fibres and periscopes
3Rough and smooth surfaces
- Rough and smooth surfaces rough surfaces scatter light in many directions, while a smooth, polished surface gives a clear reflection
4Rotating a mirror
- Rotating a mirror turning a mirror through an angle turns the reflected ray through twice that angle
Part 3 of 8Image in a plane mirror
- Plane mirror
- a flat, smooth, polished surface
| Property | Plane mirror image |
|---|---|
| Size | Same size as the object |
| Distance | Same distance behind the mirror as the object is in front |
| Type | Virtual, so it cannot be projected onto a screen |
| Orientation | Upright and laterally reversed |
Part 4 of 8Constructing the image
1
Draw a ray from the object that reflects from the mirror with i = r.
2
Draw a second ray from the object in a slightly different direction and reflect it in the same way.
3
Trace both reflected rays backwards behind the mirror with dashed lines to form virtual rays.
4
The image appears where the dashed lines cross.
- Why virtual
- the rays only appear to come from the image because they diverge, and they do not actually pass through it
Part 5 of 8Investigating reflection: variables and equipment
Aim
- Aim to investigate reflection by a plane mirror
Variables
- Variables the angle of incidence i is independent and the angle of reflection r is dependent
- Control variables are the distance of the ray box from the mirror, the width of the beam and the wavelength of the light.
| Equipment | Purpose |
|---|---|
| Ray box | Provides a narrow beam of light that is easily reflected |
| Protractor | Measures the angles of incidence and reflection |
| Sheet of paper | Marks the lines for the incident and reflected rays |
| Pencil and ruler | Draw the ray lines on the paper |
| Plane mirror | Reflects the light beam |
Part 6 of 8Investigating reflection: method
1
Set up the apparatus on a sheet of paper.
2
Draw a straight line about 10 cm long in the middle of the paper.
3
Use a protractor to draw a 90° line (the normal) that bisects the first line.
4
Stand the mirror along the first line.
5
Switch on the ray box and aim a beam at the point where the two lines cross.
6
Mark a point just after the beam leaves the ray box and a point on the reflected beam about 10 cm from the mirror.
7
Remove the ray box and mirror, then join the two marked points to the crossing point with a ruler.
8
Measure both angles from the 90° line; the ray towards the mirror gives i and the other gives r.
9
Repeat with the beam aimed at different angles, for example 10°, 30°, 45° and 80°.
- Result
- if the experiment is done correctly, i and r are the same for every angle
Part 7 of 8Evaluation and safety
| Issue | Improvement |
|---|---|
| 90° lines drawn incorrectly (systematic) | Use a set square to draw perpendicular lines |
| Distorted or blemished mirror (systematic) | Check the mirror has little or no blemishes |
| Points marked inaccurately (random) | Use a sharp pencil and mark the middle of the beam |
| Protractor resolution limits the reading (random) | Use a protractor with a higher resolution |
| Ray box can burn | Run burns under cold water for at least 5 minutes |
| Light can damage the eyes | Do not look into the light and stand behind the ray box |
| Liquids near electrical equipment | Keep liquids away from the electrical equipment and paper |
Part 8 of 8More from the specification
1Refraction and Snell's Law
- When a light ray enters a denser medium, it slows down and refracts toward the normal; the refractive index n is defined by Snell's law: n = sin(i) / sin(r).
- (such as a glass block)
2Total Internal Reflection and Critical Angle
- When light travels inside an optically dense medium towards a less dense boundary at an angle exceeding the critical angle c, the ray undergoes total internal reflection, governed by sin(c) = 1 / n.
3Converging Lenses and Principal Focus
- A converging (convex) lens refracts parallel incident rays so they meet at a focal point called the principal focus (F);
- the distance between the optical centre and principal focus is the focal length f.
4Image Formation by Lenses
- Converging lenses produce real, inverted images on a screen when the object is placed beyond the focal length,
- but produce upright, magnified virtual images when placed within the focal point.
- (acting as a magnifying glass)
5Refractive index of a block
- Refractive index of a block ratio of the sine of incidence angle to refraction angle when light enters a transparent block.
6Critical angle and reflection
- Critical angle and reflection light travelling from glass to air reflects totally internally when incidence exceeds the critical angle.
7Converging lens focal plane
- Converging lens focal plane refracted light from distant parallel rays converges cleanly at the principal focus on the focal plane.
8Refractive index
- Ratio of the speed of light in a vacuum to its speed in a material
9Critical angle
- Incline angle at which sliding just begins.
10Virtual image
- Image formed where rays only appear to meet, not projectable on a screen
11Principal focus
- Point where parallel rays meet or appear to come from
12Focal length
- Distance from the lens centre to the principal focus
13Optically dense
- Having a high refractive index
14Refraction
- Change in direction of light when it changes speed between media.
Key terms
| Term | Meaning |
|---|---|
| Normal | A line at right angles to the boundary or mirror surface. |
| Angle of incidence | Angle between the incident ray and the normal. |
| Angle of reflection | Angle between the reflected ray and the normal. |
| Plane mirror | A flat, smooth, polished reflecting surface. |
| Virtual image | An image formed where rays only appear to come from, so it cannot be projected. |
| Law of reflection | The angle of incidence equals the angle of reflection. |
Avoid these mistakes
1
Measuring the angles from the mirror surface instead of from the normal.
2
Drawing the image in front of the mirror — it is behind the mirror.
3
Saying the image is real because you can see it — it is virtual and cannot be projected.
4
Forgetting arrows on the rays.
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