Think Like a Physicist!
Reflection of Light: Understand, Apply and Explain
ICSE Class 7 | Two-Mark Thinking Questions
Observe • Think • Connect • Explain
Welcome, Young Scientists!
Have you ever raised your right hand in front of a mirror and noticed that the image appears to raise the hand on the opposite side? Have you wondered why a ray of light bounces back from a mirror or how we can predict the direction of the reflected ray? In this edition of our Think Like a Physicist series, we will explore reflection of light through everyday observations, angle calculations and thinking-based questions.Your mission: Understand the concept, identify the scientific keywords and write a precise two-mark answer.
1. The Concept Corner: What Is Reflection?
Definition: Reflection of light is the bouncing back of light into the same medium after it strikes a surface.
Examples include seeing your face in a mirror, observing trees reflected in still water and seeing your image in a polished surface.
- Incident ray: The ray of light that falls on a reflecting surface.
- Reflected ray: The ray of light that bounces back from the surface.
- Point of incidence: The point where the incident ray strikes the surface.
- Normal: An imaginary line drawn perpendicular to the reflecting surface at the point of incidence.
- Angle of incidence (i): The angle between the incident ray and the normal.
- Angle of reflection (r): The angle between the reflected ray and the normal.
2. The Two Laws of Reflection
These laws help us predict the direction in which light travels after striking a reflecting surface.
3. The Angle Trap: Surface or Normal?
The normal makes an angle of 90° with the mirror surface.
Example: An incident ray makes an angle of 30° with the mirror surface.
Angle of incidence = 90° − 30° = 60°.
According to the law of reflection, the angle of reflection is also 60°.
- Angle with the surface = 30°
- Angle of incidence = 60°
- Angle of reflection = 60°
- Angle made by the reflected ray with the surface = 30°
4. How to Write a Perfect Two-Mark Answer
Point 1: State the scientific principle or law.
Point 2: Apply the principle to explain the result or calculate the required value.
Question: A ray of light makes an angle of 30° with a plane mirror. Calculate the angle of incidence.
Model answer:
- The normal is perpendicular to the mirror surface, making an angle of 90° with it.
- Angle of incidence = 90° − 30° = 60°.
Keywords: Normal, perpendicular, angle of incidence.
5. Thinking Questions: Test Your Understanding
Define reflection of light and give one example from daily life.
Example: Seeing your image in a plane mirror.
Keywords: Bouncing back, same medium.
What is the normal? Why is it important in reflection?
It provides the reference for measuring the angles of incidence and reflection.
Keywords: Imaginary line, perpendicular, angles.
State the first law of reflection. If the angle of incidence is 35°, find the angle of reflection.
Therefore, the angle of reflection is 35°.
Keywords: Angle of incidence, angle of reflection, equal.
A ray of light makes an angle of 30° with the mirror surface. Calculate the angle of incidence and angle of reflection.
By the first law of reflection, the angle of reflection is also 60°.
Keywords: Normal, 90°, equal angles.
The angle of incidence is 40°. Calculate the angle between the incident ray and reflected ray.
Angle between the rays = 40° + 40° = 80°.
Answer: 80°.
What is lateral inversion? Give one example.
Example: The word AMBULANCE is written in reverse on the front of an ambulance so that it appears correctly oriented in rear-view mirrors.
Keywords: Left-right reversal, plane mirror.
State any two characteristics of an image formed by a plane mirror.
- The image is virtual.
- The image is erect.
- The image is the same size as the object.
- The image is laterally inverted.
- The image appears as far behind the mirror as the object is in front.
Why is the image formed by a plane mirror called virtual?
It cannot be obtained on a screen.
Keywords: Apparent meeting, cannot be obtained on a screen.
A person stands 5 cm in front of a plane mirror. How far behind the mirror is the image? Calculate the distance between the person and the image.
Distance between person and image = 5 + 5 = 10 cm.
Answer: Image distance = 5 cm; object-image distance = 10 cm.
A student says, “An image formed by a plane mirror is erect, so it cannot be laterally inverted.” Is the student correct? Explain.
Erect means upright, whereas lateral inversion means apparent left-right reversal. A plane-mirror image has both properties.
Keywords: Erect, lateral inversion.
6. Challenge Zone: Think Deeper!
The angle between the incident ray and reflected ray is 100°. Calculate the angle of incidence.
2i = 100°; therefore, i = 50°.
Answer: 50°.
A ray strikes a plane mirror along the normal. What is the angle of reflection? Explain.
By the law of reflection, the angle of reflection is also 0°. The ray retraces its path.
An object moves 2 cm towards a stationary plane mirror. How far does its image move, and by how much does the distance between the object and image change?
The object-image distance decreases by 4 cm because both the object and image move towards the mirror.
7. Quick Assessment: Test Your Thinking!
Choose the correct answer. Each question carries one mark.
8. Your Physics Answer-Writing Checklist
- Have I identified the scientific principle?
- Have I explained the reason or calculated the correct value?
- Have I used the correct scientific keywords?
- Have I measured the angle from the normal rather than the surface?
- Have I written two clear, relevant points?
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