Class 8 Science Light: Mirrors and Lenses

Class 8 Science Light: Mirrors and Lenses

1. Introduction

Light is a form of energy that allows us to see objects. Mirrors and lenses help us reflect and refract light, forming images for daily use in optics, instruments, and experiments.

2. Reflection of Light

  • Reflection: The bouncing back of light when it strikes a surface.
  • Laws of Reflection:
    1. The angle of incidence = angle of reflection
    2. The incident ray, reflected ray, and normal lie in the same plane
  • Types of Mirrors:
    1. Plane Mirror: Flat surface; forms virtual, erect images
    2. Concave Mirror: Curved inward; can form real or virtual images depending on object distance
    3. Convex Mirror: Curved outward; forms virtual, erect, diminished images

3. Characteristics of Images

PropertyPlane MirrorConcave MirrorConvex Mirror
NatureVirtual, erectReal or virtualVirtual, erect
SizeSame as objectCan be magnified or diminishedDiminished
PositionBehind the mirrorIn front or behindBehind the mirror
UsesDressing mirrorShaving, solar furnaceVehicle rear-view mirrors

4. Refraction of Light

  • Refraction: Bending of light when it passes from one medium to another (air → water, air → glass)
  • Laws of Refraction:
    1. Incident ray, refracted ray, and normal lie in the same plane
    2. Ratio of sine of angle of incidence to sine of angle of refraction is constant (Snell’s law)
  • Lenses: Transparent objects that refract light to form images
    1. Convex Lens (Converging): Thicker in middle; can form real or virtual images
    2. Concave Lens (Diverging): Thinner in middle; forms virtual, erect, diminished images

5. Focal Point and Focal Length

  • Focal Point (F): Point where parallel rays converge (convex) or appear to diverge (concave)
  • Focal Length (f): Distance between mirror/lens and focal point
  • Relation in Lenses: 1/f = 1/v – 1/u
    • u = object distance, v = image distance, f = focal length

6. Uses of Mirrors and Lenses

  • Plane Mirror: Dressing mirror, periscopes
  • Concave Mirror: Shaving mirror, torch, solar furnace
  • Convex Mirror: Vehicle rear-view mirrors, security mirrors
  • Convex Lens: Magnifying glass, spectacles for hypermetropia
  • Concave Lens: Spectacles for myopia, peepholes

7. Key Terms

TermMeaning
ReflectionBouncing back of light from a surface
RefractionBending of light when passing from one medium to another
Plane MirrorFlat mirror that forms virtual image
Concave MirrorMirror curved inward
Convex MirrorMirror curved outward
Convex LensConverging lens
Concave LensDiverging lens
Focal PointPoint where rays converge or appear to diverge
Focal LengthDistance between mirror/lens and focal point

8. Examples in Daily Life

  • Dressing mirror → Plane mirror
  • Shaving mirror → Concave mirror
  • Rear-view mirror of vehicles → Convex mirror
  • Magnifying glass → Convex lens
  • Spectacles for nearsightedness → Concave lens

9. Possible Questions

Very Short Answer Questions (1–2 marks)

  • Define reflection of light.
  • Name the type of mirror used in vehicles’ rear-view mirrors.
  • Define refraction of light.
  • Give one use of concave lens.
  • What is focal length?

Short Answer Questions (3–5 marks)

  • State laws of reflection.
  • Differentiate between concave and convex mirrors.
  • Describe convex and concave lenses with examples.
  • Explain the terms focal point and focal length.

Long Answer Questions (6–8 marks)

  • Explain image formation by concave and convex mirrors.
  • Describe reflection and refraction of light with daily life examples.
  • Draw ray diagrams showing image formation by convex and concave lenses.
  • Discuss the uses of mirrors and lenses in daily life and instruments.

Multiple Choice Questions (MCQs)

  • Plane mirror forms:
    a) Real image
    b) Virtual image
    c) Magnified image
    d) Inverted image
    Answer: b
  • Convex mirror always forms:
    a) Real, inverted image
    b) Virtual, erect image
    c) Magnified image
    d) Inverted image
    Answer: b
  • Convex lens is also called:
    a) Diverging lens
    b) Converging lens
    c) Plane lens
    d) Reflecting lens
    Answer: b

Fill in the Blanks

  • Reflection is the _______ of light from a surface.
    Answer: bouncing back
  • Refraction is the _______ of light when it passes from one medium to another.
    Answer: bending
  • The distance between lens and focal point is called _______.
    Answer: focal length

True/False Questions

  • Convex mirror forms virtual, erect images. True
  • Concave lens converges light rays. False
  • Plane mirrors form real images. False

Match the Following

Column AColumn B
Plane mirrorDressing mirror
Concave mirrorShaving mirror
Convex mirrorVehicle rear-view mirror
Convex lensMagnifying glass
Concave lensSpectacles for myopia

Reflection of Light

  1. Reflection of light is the:
    a) Bending of light
    b) Absorption of light
    c) Bouncing back of light from a surface
    d) Spreading of light
    Answer: c
  2. The angle of incidence is equal to the:
    a) Angle of deviation
    b) Angle of reflection
    c) Angle of refraction
    d) Angle of emergence
    Answer: b
  3. Incident ray, reflected ray, and normal lie in:
    a) Different planes
    b) Same plane
    c) Opposite planes
    d) Any plane
    Answer: b
  4. Plane mirrors form images that are:
    a) Real, inverted
    b) Virtual, erect
    c) Real, magnified
    d) Virtual, inverted
    Answer: b
  5. Concave mirror can form:
    a) Only virtual images
    b) Only real images
    c) Real or virtual images depending on object distance
    d) No image
    Answer: c
  6. Convex mirrors always form:
    a) Virtual, erect, diminished images
    b) Real, inverted images
    c) Magnified images
    d) No image
    Answer: a
  7. A concave mirror is also called:
    a) Diverging mirror
    b) Converging mirror
    c) Plane mirror
    d) None of these
    Answer: b
  8. A convex mirror is also called:
    a) Converging mirror
    b) Diverging mirror
    c) Plane mirror
    d) Shaving mirror
    Answer: b
  9. Image formed by a plane mirror is:
    a) Same size as object
    b) Larger than object
    c) Smaller than object
    d) Inverted only
    Answer: a
  10. A concave mirror is used in:
    a) Vehicle rear-view mirrors
    b) Dressing mirrors
    c) Shaving mirrors
    d) All of the above
    Answer: c

Refraction of Light

  1. Refraction is the:
    a) Bouncing back of light
    b) Bending of light as it passes from one medium to another
    c) Absorption of light
    d) Splitting of light into colors
    Answer: b
  2. Laws of refraction include:
    a) Angle of incidence = angle of reflection
    b) Incident ray, refracted ray, and normal lie in same plane
    c) Refraction occurs only in air
    d) None of these
    Answer: b
  3. Convex lens is a:
    a) Diverging lens
    b) Converging lens
    c) Plane lens
    d) Mirror
    Answer: b
  4. Concave lens is a:
    a) Converging lens
    b) Diverging lens
    c) Plane lens
    d) Mirror
    Answer: b
  5. Parallel rays of light converge at:
    a) Principal focus of concave mirror
    b) Principal focus of convex lens
    c) Both a and b
    d) None of these
    Answer: c
  6. Concave lens always forms:
    a) Virtual, erect, diminished images
    b) Real, inverted images
    c) Magnified images
    d) No image
    Answer: a
  7. Focal point is:
    a) Point where rays converge or appear to diverge
    b) Center of mirror
    c) Distance of object from mirror
    d) None of these
    Answer: a
  8. Focal length is:
    a) Distance between object and mirror
    b) Distance between mirror/lens and focal point
    c) Distance between image and object
    d) None of these
    Answer: b
  9. Image formed by a convex lens depends on:
    a) Focal length and object distance
    b) Size of lens only
    c) Color of object
    d) Thickness of lens only
    Answer: a
  10. Magnifying glass is a:
    a) Concave lens
    b) Convex lens
    c) Plane mirror
    d) Convex mirror
    Answer: b

Applications

  1. Rear-view mirrors of vehicles are:
    a) Concave mirrors
    b) Convex mirrors
    c) Plane mirrors
    d) Concave lenses
    Answer: b
  2. Dressing mirrors are:
    a) Plane mirrors
    b) Convex mirrors
    c) Concave mirrors
    d) Concave lenses
    Answer: a
  3. Shaving mirrors are:
    a) Plane mirrors
    b) Convex mirrors
    c) Concave mirrors
    d) Convex lenses
    Answer: c
  4. Spectacles for hypermetropia use:
    a) Concave lens
    b) Convex lens
    c) Plane lens
    d) None of these
    Answer: b
  5. Spectacles for myopia use:
    a) Concave lens
    b) Convex lens
    c) Plane lens
    d) None of these
    Answer: a
  6. Solar furnace uses:
    a) Convex mirror
    b) Concave mirror
    c) Plane mirror
    d) Concave lens
    Answer: b
  7. Periscope uses:
    a) Plane mirrors
    b) Convex mirrors
    c) Concave mirrors
    d) Lenses only
    Answer: a
  8. Focusing sunlight to burn paper is done with:
    a) Convex lens
    b) Concave mirror
    c) Plane mirror
    d) Concave lens
    Answer: a
  9. Convex lens forms real image when:
    a) Object is beyond focus
    b) Object is at focus
    c) Object is between lens and focus
    d) Object is at infinity
    Answer: a
  10. Convex lens forms virtual image when:
    a) Object is beyond focus
    b) Object is at focus
    c) Object is between lens and focus
    d) Object is at infinity
    Answer: c

Ray Diagrams & Image Characteristics

  1. Image formed by plane mirror is:
    a) Inverted
    b) Erect
    c) Real
    d) Both real and inverted
    Answer: b
  2. Concave mirror forms magnified image when:
    a) Object is beyond focus
    b) Object is between focus and mirror
    c) Object is at infinity
    d) Object is at focus
    Answer: b
  3. Convex mirror forms:
    a) Magnified image
    b) Diminished image
    c) Real image
    d) Inverted image
    Answer: b
  4. Principal focus of concave mirror lies:
    a) In front of mirror
    b) Behind mirror
    c) On object
    d) On image
    Answer: a
  5. Principal focus of convex mirror lies:
    a) In front of mirror
    b) Behind mirror
    c) On object
    d) None
    Answer: b
  6. Real image is:
    a) Erect
    b) Formed on screen
    c) Virtual
    d) Always diminished
    Answer: b
  7. Virtual image is:
    a) Formed on screen
    b) Cannot be formed on screen
    c) Always inverted
    d) Only for lenses
    Answer: b
  8. Concave lens always forms:
    a) Virtual, erect, diminished image
    b) Real, inverted image
    c) Magnified image
    d) None
    Answer: a
  9. A convex lens at 2F forms:
    a) Image at 2F
    b) Image at F
    c) Image between F and 2F
    d) Image at infinity
    Answer: a
  10. A concave mirror forms magnified image when object is:
    a) Beyond C (center of curvature)
    b) Between F and mirror
    c) At C
    d) At infinity
    Answer: b

True/False

  1. Plane mirror forms virtual, erect images. True
  2. Convex mirror can form real image. False
  3. Concave mirror can form both real and virtual images. True
  4. Convex lens is diverging lens. False
  5. Focal point is the point where rays converge or appear to diverge. True

Fill in the Blanks

  1. Laws of reflection include angle of incidence = _______.
    Answer: angle of reflection
  2. Convex mirror always forms _______ images.
    Answer: virtual, erect, diminished
  3. Concave lens always forms _______ images.
    Answer: virtual, erect, diminished
  4. Distance between lens/mirror and focal point is called _______.
    Answer: focal length
  5. Dressing mirror uses a _______ mirror.
    Answer: plane