Cambridge IGCSE Physics 0625 – 3.2.1 Reflection of Light3.2.1 Reflection of Light
Objective
Use simple constructions, measurements and calculations to investigate reflection by plane mirrors.
Key Concepts
- The incident ray, reflected ray and the normal are all in the same plane.
- Law of Reflection: the angle of incidence equals the angle of reflection (\$\thetai = \thetar\$).
- A plane mirror produces a virtual image that is upright, laterally inverted and the same distance behind the mirror as the object is in front of it.
Ray Diagram Construction
- Draw the mirror as a straight line and mark a point on the line as the point of incidence.
- Draw the normal at the point of incidence (a line perpendicular to the mirror).
- From the object, draw the incident ray to the point of incidence.
- Measure the angle between the incident ray and the normal (\$\theta_i\$).
- From the point of incidence, draw the reflected ray on the opposite side of the normal such that \$\thetar = \thetai\$.
- Extend the reflected ray backward (dotted line) to locate the virtual image.
Suggested diagram: Ray diagram for a plane mirror showing incident ray, normal, reflected ray and virtual image.
Measurements
When performing an experiment with a plane mirror, record the following quantities for each trial:
| Trial | Object distance, \$d_o\$ (cm) | Angle of incidence, \$\theta_i\$ (°) | Angle of reflection, \$\theta_r\$ (°) | Image distance, \$d_i\$ (cm) |
|---|
| 1 | | | | |
| 2 | | | | |
| 3 | | | | |
Calculations
For a plane mirror the relationship between object and image distances is:
\$di = -do\$
The negative sign indicates that the image is virtual and located behind the mirror.
To verify the law of reflection, calculate the difference between the measured angles:
\$\Delta\theta = |\thetai - \thetar|\$
A small \$\Delta\theta\$ (typically < 1°) confirms the law.
Example Problem
Given: An object is placed 30 cm in front of a plane mirror. The incident ray makes an angle of \$35^\circ\$ with the normal.
Find:
- The angle of reflection.
- The position of the image relative to the mirror.
Solution:
- By the law of reflection, \$\thetar = \thetai = 35^\circ\$.
- The image distance is \$di = -do = -30\text{ cm}\$. The image is 30 cm behind the mirror, directly opposite the object.
Common Errors and How to Avoid Them
- Measuring angles from the mirror surface instead of the normal. Always draw and use the normal as the reference line.
- Confusing virtual and real images. A virtual image cannot be projected onto a screen; it appears behind the mirror.
- Incorrect sign convention for distances. Use a negative sign for image distances in plane mirror calculations.
Summary
- The incident ray, reflected ray and normal lie in the same plane.
- Law of reflection: \$\thetai = \thetar\$.
- Plane mirrors produce upright, laterally inverted virtual images at the same distance behind the mirror as the object is in front.
- Accurate construction of ray diagrams and careful measurement of angles verify the law of reflection.
Practice Questions
- A student shines a laser beam at a plane mirror such that the incident angle is \$20^\circ\$. What is the angle of reflection?
- An object is 45 cm in front of a plane mirror. Where is the image formed? State its nature (real/virtual, upright/inverted).
- During an experiment, the measured angles are \$\thetai = 40^\circ\$ and \$\thetar = 42^\circ\$. Calculate \$\Delta\theta\$ and comment on the result.
- Explain why a plane mirror does not change the size of the image, using the geometry of the ray diagram.