CIE iGCSE Co-ordinated Sciences-P3.2.1 Reflection of light- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-P3.2.1 Reflection of light – Study Notes
CIE iGCSE Co-ordinated Sciences-P3.2.1 Reflection of light – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
1. Use ray diagrams to define the terms normal, angle of incidence and angle of reflection
2. Describe the formation of an optical image by a vertical plane mirror and give its characteristics compared with the object, i.e. same size, same distance from mirror, laterally inverted
3. State that for reflection, the angle of incidence is equal to the angle of reflection; recall and use this relationship
Supplement
4. Describe the formation of an optical image by a plane mirror and explain why it is virtual
5. Use simple diagrams, measurements and calculations for reflection by plane mirrors
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Reflection at a Plane Surface
(a) Key Terms with Ray Diagram
When a wave (e.g. light) strikes a flat surface such as a mirror, the following terms are defined:
- Point of incidence: The exact point on the surface where the incident ray strikes.
- Normal: An imaginary line drawn perpendicular (at \(90^\circ\)) to the surface at the point of incidence.
- Incident ray: The ray of light approaching the reflecting surface.
- Reflected ray: The ray of light leaving the surface after reflection.
- Angle of incidence (\(\theta_i\)): The angle between the incident ray and the normal.
- Angle of reflection (\(\theta_r\)): The angle between the reflected ray and the normal.
(b) Law of Reflection
- The law of reflection states: $ \theta_i = \theta_r $
- The incident ray, reflected ray, and the normal all lie in the same plane.
- The speed, frequency, and wavelength of the wave remain unchanged during reflection only the direction of the wave changes.
- Reflection can occur for different types of waves: light waves (mirrors), sound waves (echo), and water waves (bouncing off barriers).
Real-life Example:
- A mirror producing an image.
- An echo produced when sound reflects from a large wall or cliff.
Example
A ray of light strikes a plane mirror at an angle of incidence of \( 40^\circ \). Calculate the angle of reflection, and state whether the speed of light changes after reflection.
▶️Answer/Explanation
Step (1) – Recall law of reflection:
\(\theta_i = \theta_r\).
Step (2) – Substitute values:
\(\theta_r = \theta_i = 40^\circ\).
Step (3) – Wave properties after reflection:
The light wave remains in the same medium (air). Therefore, speed, frequency, and wavelength do not change.
Final Answer:
(a) Angle of reflection = \( 40^\circ \). (b) Speed of light remains the same.
Image Formation by Plane Mirrors
(a) Formation of an Optical Image by a Vertical Plane Mirror
When light rays from an object strike a plane mirror, they reflect according to the law of reflection. The reflected rays appear to come from a point behind the mirror, forming an optical image.
- The image is formed by extending the reflected rays backwards.
- The rays do not actually meet behind the mirror → the image is virtual.
(b) Characteristics of the Image
- Same size as the object.
- Appears at the same distance behind the mirror as the object is in front.
- Laterally inverted (left–right reversal).
- Virtual (cannot be projected on a screen).
- Upright (not upside down).
(c) Why the Image is Virtual
- The reflected rays diverge in front of the mirror.
- They only appear to meet when extended behind the mirror.
- No real light rays pass through the image position → cannot be formed on a screen.
(d) Measurements and Calculations
- If object distance is \( d_o \), image distance is \( d_i = d_o \).
- If object height is \( h_o \), image height is \( h_i = h_o \).
- Magnification: $M = \dfrac{h_i}{h_o} = \dfrac{d_i}{d_o} = 1 $
Example
An object \( 6~\text{cm} \) tall is placed \( 10~\text{cm} \) in front of a plane mirror. Find the position, size, and nature of the image formed.
▶️Answer/Explanation
Step (1) – Position of image:
The image forms the same distance behind the mirror:
\( d_i = d_o = 10~\text{cm} \).
Step (2) – Size of image:
\( h_i = h_o = 6~\text{cm} \).
Step (3) – Nature of image:
Same size as object.
Same distance behind mirror as object is in front.
Virtual and upright.
Laterally inverted.
Final Answer:
Image height = \( 6~\text{cm} \), distance behind mirror = \( 10~\text{cm} \), nature = virtual, upright, laterally inverted.