Light - Reflection and Refraction
Understanding the behavior of light through reflection and refraction, and their applications in mirrors and lenses
Introduction to Light
Light is a form of energy that enables us to see objects. We see objects when light reflected from them enters our eyes. Light typically travels in straight lines, but it can change direction when it encounters different media through reflection and refraction.
Key Concepts
- Light travels in straight lines (rectilinear propagation)
- Objects become visible when light reflects off them and enters our eyes
- Light can be reflected (bounced off surfaces) and refracted (bent when passing through different media)
- These phenomena explain many optical effects in nature
Nature of Light
Light exhibits dual nature - it behaves both as waves and particles. For most everyday phenomena studied in this chapter, we consider light as traveling in straight lines (ray optics). However, at very small scales, light shows wave properties (diffraction) and particle properties (photoelectric effect).
Learning Outcomes
After studying this chapter, students will be able to:
- Understand the laws of reflection and refraction
- Differentiate between concave and convex mirrors
- Draw ray diagrams for image formation by spherical mirrors
- Apply mirror formula to solve numerical problems
- Understand refraction through rectangular glass slabs
- Differentiate between convex and concave lenses
- Draw ray diagrams for image formation by lenses
- Apply lens formula to solve numerical problems
- Calculate power of lenses and understand its significance
Period-Wise Teaching Plan
This chapter is designed to be covered over 10 periods, each lasting 45 minutes. Below is the detailed period-wise plan:
Topics: Nature of light, rectilinear propagation, laws of reflection.
Activities: Discussion on how we see objects, demonstration of light traveling in straight lines.
Topics: Concave and convex mirrors, terminology (pole, center of curvature, focal length, etc.).
Activities: Activity 9.1 - Observing images in curved spoon surfaces.
Topics: Image formation by concave mirrors, ray diagrams.
Activities: Activity 9.2 - Finding focal length of concave mirror using sunlight.
Topics: Image formation by convex mirrors, uses of spherical mirrors.
Activities: Activity 9.5 - Observing images in convex mirrors.
Topics: Sign convention, mirror formula, magnification, numerical problems.
Activities: Solving numerical problems, practice with different object positions.
Topics: Laws of refraction, refractive index, apparent depth.
Activities: Activity 9.7 - Picking coin from water, Activity 9.8 - Disappearing coin illusion.
Topics: Refraction through rectangular glass slab, lateral displacement.
Activities: Activity 9.9 - Observing bending of line through glass slab.
Topics: Convex and concave lenses, terminology, focal length.
Activities: Activity 9.11 - Finding focal length of convex lens using sunlight.
Topics: Image formation by lenses, ray diagrams, lens formula.
Activities: Activity 9.12 - Image formation by convex lens for different object positions.
Topics: Lens formula, magnification, power of lenses, numerical problems.
Activities: Solving numerical problems, understanding lens power in spectacles.
Teaching Methodology
The teaching approach for this chapter should be a blend of:
- Demonstrations and hands-on activities
- Ray diagram drawing practice
- Numerical problem solving
- Real-world applications of mirrors and lenses
- Group discussions on optical phenomena
- Regular assessment through quizzes and worksheets
Reflection of Light
Reflection is the phenomenon where light bounces off a surface when it encounters that surface. The laws of reflection govern how light behaves when it reflects off surfaces.
Laws of Reflection
- The angle of incidence is equal to the angle of reflection
- The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane
Image Formation by Plane Mirrors
Plane mirrors form virtual, erect images that are:
- The same size as the object
- As far behind the mirror as the object is in front
- Laterally inverted (left-right reversed)
Place two mirrors at right angles to each other and observe how many images are formed. Change the angle between mirrors and observe the change in number of images.
Construct a simple periscope using two plane mirrors placed at 45° angles to demonstrate the principle of reflection in periscopes.
Spherical Mirrors
Spherical mirrors are mirrors whose reflecting surface is part of a hollow sphere. They can be concave (converging) or convex (diverging).
Terminology of Spherical Mirrors
- Pole (P): The center of the mirror surface
- Center of Curvature (C): The center of the sphere of which the mirror is a part
- Radius of Curvature (R): The radius of the sphere (distance from C to P)
- Principal Axis: The line passing through P and C
- Principal Focus (F): The point where parallel rays converge or appear to diverge from
- Focal Length (f): The distance from P to F (f = R/2)
- Aperture: The diameter of the mirror's reflecting surface
Image Formation by Concave Mirrors
| Position of Object | Position of Image | Size of Image | Nature of Image |
|---|---|---|---|
| At infinity | At F | Highly diminished | Real, inverted |
| Beyond C | Between F and C | Diminished | Real, inverted |
| At C | At C | Same size | Real, inverted |
| Between C and F | Beyond C | Enlarged | Real, inverted |
| At F | At infinity | Highly enlarged | Real, inverted |
| Between P and F | Behind the mirror | Enlarged | Virtual, erect |
Image Formation by Convex Mirrors
Convex mirrors always form virtual, erect, and diminished images regardless of the object's position.
The mirror formula relates object distance (u), image distance (v), and focal length (f):
1/v + 1/u = 1/f
Where:
- u is always negative (object is always in front of mirror)
- v is negative for real images, positive for virtual images
- f is negative for concave mirrors, positive for convex mirrors
Magnification (m) = height of image / height of object = -v/u
Where:
- Positive m indicates erect image
- Negative m indicates inverted image
- |m| > 1 indicates enlarged image
- |m| < 1 indicates diminished image
- |m| = 1 indicates same size image
Using a concave mirror, candle, and screen, observe image formation for different object positions and verify the results in the table above.
Compare the field of view of plane, concave, and convex mirrors to understand why convex mirrors are used as rear-view mirrors.
Refraction of Light
Refraction is the bending of light when it passes from one transparent medium to another. This occurs because light travels at different speeds in different media.
Laws of Refraction
- The incident ray, the refracted ray, and the normal to the interface at the point of incidence all lie in the same plane
- The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media (Snell's Law)
Refractive Index
The refractive index (n) of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in that medium:
n = c/v
Where c is the speed of light in vacuum (3 × 10⁸ m/s) and v is the speed of light in the medium.
Relative Refractive Index
The refractive index of medium 2 with respect to medium 1 is given by:
¹n₂ = n₂/n₁ = v₁/v₂ = sin i / sin r
Refraction through Rectangular Glass Slab
When light passes through a rectangular glass slab:
- It bends towards the normal when entering the glass (from air to glass)
- It bends away from the normal when exiting the glass (from glass to air)
- The emergent ray is parallel to the incident ray but laterally displaced
Apparent Depth
Due to refraction, objects in water appear shallower than they actually are. The apparent depth (d') is related to the real depth (d) by:
d' = d/n
Where n is the refractive index of water.
Try to pick a coin from the bottom of a bucket filled with water. Observe how refraction makes this difficult and requires aiming at a different spot.
Place a coin in an empty bowl and move back until it just disappears from view. Then pour water into the bowl and observe how the coin becomes visible again due to refraction.
Use pins to trace the path of light through a glass slab and verify the laws of refraction.
Lenses
Lenses are transparent optical devices with curved surfaces that refract light to form images. They can be converging (convex) or diverging (concave).
Terminology of Lenses
- Optical Center (O): The center of the lens
- Principal Axis: The line passing through O and the centers of curvature
- Principal Focus (F): The point where parallel rays converge or appear to diverge from
- Focal Length (f): The distance from O to F
- Aperture: The diameter of the lens
Image Formation by Convex Lenses
| Position of Object | Position of Image | Size of Image | Nature of Image |
|---|---|---|---|
| At infinity | At F₂ | Highly diminished | Real, inverted |
| Beyond 2F₁ | Between F₂ and 2F₂ | Diminished | Real, inverted |
| At 2F₁ | At 2F₂ | Same size | Real, inverted |
| Between F₁ and 2F₁ | Beyond 2F₂ | Enlarged | Real, inverted |
| At F₁ | At infinity | Highly enlarged | Real, inverted |
| Between F₁ and O | Same side as object | Enlarged | Virtual, erect |
Image Formation by Concave Lenses
Concave lenses always form virtual, erect, and diminished images regardless of the object's position.
The lens formula relates object distance (u), image distance (v), and focal length (f):
1/v - 1/u = 1/f
Where:
- u is always negative (object is always in front of lens)
- v is negative for real images, positive for virtual images
- f is positive for convex lenses, negative for concave lenses
Magnification (m) = height of image / height of object = v/u
Where:
- Positive m indicates erect image
- Negative m indicates inverted image
- |m| > 1 indicates enlarged image
- |m| < 1 indicates diminished image
- |m| = 1 indicates same size image
Power of a Lens
The power of a lens measures its ability to converge or diverge light rays. It is defined as the reciprocal of the focal length (in meters):
P = 1/f
Where:
- Power is measured in diopters (D)
- Convex lenses have positive power
- Concave lenses have negative power
Combination of Lenses
When multiple lenses are placed in contact, the combined power is the algebraic sum of individual powers:
P = P₁ + P₂ + P₃ + ...
Use sunlight to find the focal length of a convex lens by focusing sunlight to a point on paper and measuring the distance.
Using a convex lens, candle, and screen, observe image formation for different object positions and verify the results in the table above.
Observe that a concave lens always forms virtual, erect, and diminished images regardless of object position.
Teaching Resources
Key Terms
- Reflection: Bouncing back of light from a surface
- Refraction: Bending of light when passing from one medium to another
- Concave mirror: Converging mirror with inward curved surface
- Convex mirror: Diverging mirror with outward curved surface
- Convex lens: Converging lens thicker at the center
- Concave lens: Diverging lens thinner at the center
- Focal length: Distance from pole/optical center to principal focus
- Refractive index: Ratio of speed of light in vacuum to speed in medium
- Power of lens: Reciprocal of focal length (in meters)
- Diopter: Unit of power of lens
- Real image: Image formed where light actually converges
- Virtual image: Image formed where light appears to diverge from
Assessment Questions
Chapter Review Questions
- Define the principal focus of a concave mirror.
- The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
- Name a mirror that can give an erect and enlarged image of an object.
- Why do we prefer a convex mirror as a rear-view mirror in vehicles?
- A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
- Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 10⁴ m/s.
- Find out, from Table 9.3, the medium having highest optical density. Also find the medium with lowest optical density.
- You are given kerosene, turpentine and water. In which of these does the light travel fastest?
- The refractive index of diamond is 2.42. What is the meaning of this statement?
- Define 1 dioptre of power of a lens.
- A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
- Find the power of a concave lens of focal length 2 m.
Additional Resources
- Interactive ray diagram simulators for mirrors and lenses
- 3D models of light reflection and refraction
- Videos demonstrating optical illusions due to refraction
- Problem-solving worksheets with numerical problems
- Printable diagrams for labeling practice
- Real-world examples of mirrors and lenses applications
