CBSE Class 7 Science Light, Reflection and Plane Mirrors Worksheet Set 10

Answer: A virtual image is an image that cannot be obtained on a screen. It appears to be behind the mirror, as in a plane mirror.

Teacher's Note:
1. The reflected rays only appear to meet; they do not actually meet.
2. You can see such an image but cannot touch it.

Answer: A real image is an image that can be obtained on a screen because the rays actually meet at the image position.

Teacher's Note:
1. Real images are inverted.
2. A virtual image is never formed on a screen.

Answer: 1. The image is virtual and erect.
2. It is of the same size as the object.
3. It is formed as far behind the mirror as the object is in front.
4. It is laterally inverted (left and right appear interchanged).

Teacher's Note:
1. If you raise your right hand, the image seems to raise its left hand.
2. This is called lateral inversion.

Answer: When parallel rays fall on a smooth, polished surface and are reflected as parallel rays in one direction, it is called regular reflection.

Teacher's Note:
1. A plane mirror shows regular reflection.
2. Clear images are formed only in this case.

Answer: When parallel rays fall on a rough surface and are reflected in many different directions, it is called irregular reflection.

Teacher's Note:
1. The surface looks smooth to us but is uneven on a small scale.
2. No clear image is formed.

Answer: (a) Regular reflection from the smooth glass surface. (b) Regular reflection from the polished metal sheet. (c) Irregular reflection from crumpled foil, because its surface is uneven.

Teacher's Note:
1. Smooth, polished surfaces reflect regularly.
2. Rough or crumpled surfaces scatter the light.

Answer: (c) A is true but R is false. A leaf reflects green light and absorbs most of the other colours, so Reason is false.

Teacher's Note:
1. A coloured object appears to have the colour it reflects.
2. The statement in R is the opposite of what happens.

Answer: (a) Both A and R are true and R is the correct explanation of A. White clothes reflect most of the light and heat falling on them, so they absorb less and keep us cooler.

Teacher's Note:
1. Reflection of light explains the cooler feeling.
2. Dark clothes absorb more and feel warmer.

Answer: (b) Both A and R are true but R is not the correct explanation of A. We see him because light reflected from him enters our eyes. Straight-line travel is true but is not the explanation.

Teacher's Note:
1. Light must be reflected from an object into the eye for us to see it.
2. Straight-line travel only explains the path.

Answer: (c) A is true but R is false. The flash is seen first because light travels much faster than sound, so the Reason is false.

Teacher's Note:
1. Light covers about 3 × 10⁸ m every second.
2. Sound travels only about 340 m every second in air.

Answer: 1. The angle of incidence is equal to the angle of reflection.
2. The incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

Teacher's Note:
1. Angles are measured from the normal.
2. These laws hold for every type of reflecting surface.

Answer: Angle of incidence = 40°, so angle of reflection = 40°. The angle between the incident and reflected rays = 40° + 40° = 80°.

Teacher's Note:
1. Both angles are measured from the normal.
2. The rays lie on opposite sides of the normal.

Answer: The image is 2.5 m behind the mirror, so the distance between her and her image = 2.5 + 2.5 = 5 m.

Teacher's Note:
1. In a plane mirror the image distance equals the object distance.
2. The distance to the image is measured from the person.

Answer: The driver of a vehicle ahead sees it in the rear-view mirror, where lateral inversion makes the word read correctly.

Teacher's Note:
1. A mirror interchanges left and right.
2. Writing it reversed makes the image readable.

Answer: (b) Sun. The sun produces its own light. The others only reflect light.

Teacher's Note:
1. Luminous objects are sources of light.
2. Non-luminous objects are seen by reflected light.

Answer: (c) polished metal sheet. A polished surface gives regular reflection, which is needed for clear images.

Teacher's Note:
1. Rough surfaces scatter the reflected rays.
2. So they cannot form images.

Answer: (b) blocks the path of light. Light travels in straight lines and cannot pass through an opaque object, leaving a dark region behind it.

Teacher's Note:
1. The shadow shape depends on the object and the light source.
2. This also shows that light travels in straight lines.

Answer: (a) angle of incidence. This is the first law of reflection.

Teacher's Note:
1. Both angles are measured from the normal to the surface.
2. They are always equal.