Light – Reflection and Refraction · Lesson 1 of 15
Reflection of Light
“Light follows rules so faithfully it could teach the class punctuality.”
• Explain how reflected light enables us to see objects. • State and interpret the laws of reflection. • Identify the incident ray, normal, reflected ray and their angles. • Describe the image formed by a plane mirror. • Relate the two surfaces of a shining spoon to curved mirrors.
Walk into a completely dark room and familiar objects seem to disappear. The objects have not gone anywhere; light from them is simply not reaching your eyes. When a lamp is switched on, light falls on the objects and some of it returns toward your eyes. This return of light is the starting point for understanding reflection.
In a uniform transparent medium, light is treated as travelling along straight lines. A narrow straight path used to represent the direction of travel is called a ray of light. Sharp shadows produced by small sources support this ray model. If an obstacle becomes extremely small, however, light can bend around its edges. This effect, called diffraction, cannot be explained by straight rays and requires a wave description of light.
The phenomenon in which light falling on a surface returns into the same medium.
Laws of Reflection
At the point where a ray meets a reflecting surface, an imaginary line is drawn perpendicular to the surface. This line is the normal. The angle between the incident ray and the normal is the angle of incidence, while the angle between the reflected ray and the normal is the angle of reflection. Angles are measured from the normal, not from the mirror.
- The angle of incidence is equal to the angle of reflection.
- The incident ray, reflected ray and normal at the point of incidence lie in the same plane.
Problem
A ray makes an angle of 35° with a plane mirror. Find its angle of incidence and angle of reflection.
- 1.The given angle is measured from the mirror, but the angle of incidence is measured from the normal.
- 2.The normal is perpendicular to the mirror, so the angle of incidence is 90° - 35° = 55°.
- 3.By the first law of reflection, the angle of reflection is also 55°.
- 4.Check: each ray makes 35° with the mirror and 55° with the normal, so the complementary angles are consistent.
Image Formed by a Plane Mirror
The reflected rays from a plane mirror do not actually meet behind it. Your eye traces them backward in straight lines, so they appear to come from a point behind the mirror. The image is therefore virtual and cannot be caught on a screen. It is erect, the same size as the object, and as far behind the mirror as the object is in front. It is also laterally inverted: left and right appear interchanged.
| Property | Plane-mirror image |
|---|---|
| Nature | Virtual and erect |
| Size | Same as the object |
| Position | Equal distance behind the mirror |
| Orientation | Laterally inverted |
Observing Curved Reflecting Surfaces
A polished spoon provides two curved reflecting surfaces. The hollow inner side can produce an enlarged erect image when the face is close, but the image becomes inverted when the spoon is moved farther away. The bulging outer side gives an erect, diminished image over a wide range of distances. These changes show that image formation depends on both the curvature of the surface and the object position.
Activity
Purpose: Compare images formed by the two surfaces of a shining spoon. Hold a clean spoon close to your face and observe the image in its inner surface. Move it slowly away and note changes in size and orientation. Turn the spoon around and repeat with the outer surface. Record whether each image is erect or inverted and enlarged or diminished. The inner and outer surfaces behave like concave and convex mirrors respectively.
Quiz
Why is an object not visible in a completely dark room?
A ray makes 20° with the normal. What is the angle of reflection?
Which property is not true of an ideal plane-mirror image?
Why are reflection angles measured from the normal?
The inner surface of a spoon most closely behaves like which mirror?
Practice Problems
- A ray makes 48° with a plane mirror. Find the angles of incidence and reflection. Solution: The angle with the normal is 90° - 48° = 42°. Therefore i = r = 42°.
- An object is 18 cm in front of a plane mirror. Where is its image, and what is the separation between object and image? Solution: The image is 18 cm behind the mirror. Separation = 18 cm + 18 cm = 36 cm.
- Explain why a plane-mirror image cannot be obtained on a screen. Solution: Reflected rays do not actually meet behind the mirror; only their backward extensions appear to meet. Since no light converges at the image position, a screen cannot receive the image.
- A student says a ray making 30° with the mirror has an incidence angle of 30°. Correct the statement. Solution: Incidence is measured from the normal. The correct angle is 90° - 30° = 60°.
- Compare the images seen in the inner and outer surfaces of a spoon when the face is close. Solution: The inner concave surface can give an enlarged erect image when the face is within its focus. The outer convex surface gives an erect diminished image.
Key Takeaways
• Objects become visible when light reflected from them reaches the eyes. • Light is represented by straight rays when diffraction can be neglected. • The angle of incidence equals the angle of reflection. • All reflection angles are measured from the normal. • A plane mirror forms a virtual, erect, same-sized and laterally inverted image. • The inner and outer surfaces of a spoon behave approximately like concave and convex mirrors.
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Spherical Mirrors