P3.2.1P3. Waves
Reflection of light — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 3 of 9 1 quiz
Lesson summary
P3.2.1 Reflection of light — COMBINED SCIENCE notes: State the law of reflection, describe the image in a plane mirror and describe how to investigate reflection.Normal = a line drawn at right angles to the boundary or mirror at the point where the ray strikes it.Angle of inciden…
- Angle of incidence (i)
- Angle of reflection (r)
- Measuring angles
- Ray along the normal
- Unchanged properties
- Virtual image
Section 1 of 8Ray diagram terms for reflection
Normal
Normal a line drawn at right angles to the boundary or mirror at the point where the ray strikes it.
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 the incident ray points towards the mirror and the reflected ray points away from it.
Measuring angles
- Measuring angles angles are always measured from the normal, not from the mirror surface.
Section 2 of 8Law of reflection
Statement
- Statement the angle of incidence equals the angle of reflection, so i = r.
Ray along the normal
- Ray along the normal a ray with i = 0° reflects straight back along itself.
Unchanged properties
- Unchanged properties speed, frequency and wavelength of the light do not change on reflection.
Uses
- Uses mirrors, cameras, optical fibres and periscopes.
Section 3 of 8Image in a plane mirror
Image in a plane mirror
Plane mirror
Plane mirror a flat, smooth, polished reflecting surface.
Size
- Size the image is the same size as the object.
Distance
- Distance the image is as far behind the mirror as the object is in front of it.
Type
- Type the image is virtual and upright, and it is laterally inverted.
- (left and right swap)
Virtual image
- Virtual image the rays only appear to come from the image and do not pass through it, so it cannot be projected onto a screen.
Section 4 of 8Constructing the image in a plane mirror
1
Draw two rays from the same point on the object to the mirror.
2
At each point of incidence draw the normal and the reflected ray with i = r.
3
Extend each reflected ray backwards behind the mirror with a dashed line.
4
Mark the image where the two dashed lines cross.
Section 5 of 8Investigating reflection
1
Draw a line about 10 cm long on a sheet of paper and draw a 90° line (the normal) through its middle with a protractor or set square.
2
Place a plane mirror on the first line.
3
Aim a narrow ray from a ray box at the point where the lines cross.
4
Mark two points on the incident ray and two on the reflected ray.
5
Remove the mirror, join the points with a ruler and measure the angles from the normal.
6
Repeat for several different angles of incidence.
Result
- Result the angle of reflection equals the angle of incidence every time.
Reducing errors
- Reducing errors use a set square for the normal, a sharp pencil, and a mirror without blemishes.
Safety
- Safety the ray box can burn, so do not touch it, and do not look directly into the beam.
Section 6 of 8More from the specification
More from the specification
Law of reflection
- Law of reflection the angle of incidence equals the angle of reflection, both measured from the normal.
Refraction
- Refraction light bends at a boundary as its speed changes; the refractive index gives n = sin i over sin r.
Critical angle
- Critical angle the incident angle in a dense medium where the refracted ray travels along the boundary;
- beyond it total internal reflection occurs.
Lenses
- Lenses converging lenses bring parallel rays to the focal point, while diverging lenses spread them apart.
Images
- Images real images can be projected on a screen, virtual images cannot; a glass block shifts a ray sideways.
Law of reflection
- Law of reflection the angle of incidence equals the angle of reflection, both measured from the normal.
Refraction
- Refraction light bends at a boundary as its speed changes; the refractive index gives n = sin i over sin r.
Critical angle
- Critical angle the incident angle in a dense medium where the refracted ray travels along the boundary;
- beyond it total internal reflection occurs.
Lenses
- Lenses converging lenses bring parallel rays to the focal point, while diverging lenses spread them apart.
Images
- Images real images can be projected on a screen, virtual images cannot; a glass block shifts a ray sideways.
Refraction
- The change of direction of light when it passes between materials.
Refractive index
- The ratio of the speed of light in a vacuum to its speed in a material, n = c ÷ v.What the symbols mean: n = refractive index (no unit) · c = speed of light in a vacuum (3.00 × 108 m s−1) · v = speed of light in the material (m s−1)
Critical angle
- The angle of incidence in the denser medium at which the refracted ray travels along the boundary.
Principal focus
- The point where rays parallel to the principal axis meet or appear to come from.
Focal length
- The distance from the centre of the lens to the principal focus.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Normal | A line at right angles to the surface at the point where a ray meets it. |
| Angle of incidence | The angle between the incident ray and the normal. |
| Angle of reflection | The angle between the reflected ray and the normal. |
| Law of reflection | The angle of incidence equals the angle of reflection. |
| Virtual image | An image formed where rays only appear to come from, which cannot be projected. |
| Plane mirror | A flat, smooth, polished mirror. |
Section 8 of 8Exam technique
1
Measuring angles from the mirror surface — measure from the normal.
2
Saying the plane mirror image is real — it is virtual and cannot be projected.
3
Forgetting lateral inversion — left and right are reversed in the image.
4
Saying the image is further behind the mirror than the object is in front — the distances are equal.
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