In a concave mirror, the focus lies behind the mirror.
Ans: False (Focus lies in front of a concave mirror).
Radius of curvature (R) is twice the focal length (f).
Ans: True
A convex mirror always forms a diminished image.
Ans: True
Dentists use convex mirrors to see the teeth of patients.
Ans: False (They use concave mirrors).
A concave mirror can produce both real and virtual images.
Ans: True
Snell’s law is related to reflection of light.
Ans: False (It is related to refraction of light).
When light enters from air to glass, it bends away from the normal.
Ans: False (It bends towards the normal).
Refractive index is the ratio of speed of light in vacuum to that in the medium.
Ans: True
Convex lens is also known as converging lens.
Ans: True
Concave lens always produces a real image.
Ans: False (It always produces virtual, erect and diminished image).
The SI unit of power of a lens is Dioptre.
Ans: True
A convex mirror gives a wider field of view than a plane mirror.
Ans: True
Power of a concave lens is always negative.
Ans: True
Q1. Explain the laws of reflection of light with an example.
Ans: The two laws of reflection state: (i) The angle of incidence equals the angle of reflection. (ii) The incident ray, reflected ray, and normal at the point of incidence all lie in the same plane. Plane mirrors follow these laws.
Q2. Differentiate between concave and convex mirrors.
Ans: A concave mirror has reflecting surface curved inward and can form real or virtual images. Convex mirror has reflecting surface curved outward, always gives virtual, erect, and diminished images, and is used as a rear-view mirror in vehicles.
Q3. Why are concave mirrors used in solar furnaces?
Ans: Concave mirrors converge parallel rays of the Sun at their focus, producing a lot of heat. This concentrated energy can ignite objects and is used in solar furnaces for generating high temperature for cooking or heating.
Q4. Explain the principal focus of a convex mirror.
Ans: In a convex mirror, parallel rays appear to diverge after reflection. These diverging rays, if extended backwards, meet at a point on the principal axis. This point is called the principal focus of the convex mirror.
Q5. What is refraction of light? Give an example.
Ans: Refraction is the bending of light when it passes obliquely from one transparent medium to another due to change in its speed. Example: A pencil partly immersed in water appears bent at the surface.
Q6. What is meant by refractive index of a medium?
Ans: Refractive index of a medium is the ratio of speed of light in vacuum (or air) to the speed of light in that medium. It indicates optical density. Higher refractive index means light travels slower in that medium.
Q7. Describe the image formation by a concave lens.
Ans: A concave lens always forms a virtual, erect, and diminished image, no matter where the object is placed. The image is formed on the same side of the lens as the object, and it cannot be captured on a screen.
Q8. Why are convex lenses called converging lenses?
Ans: A convex lens is thicker at the centre and thinner at edges. It bends parallel rays of light towards each other and brings them to a single point, called the principal focus. Hence, it is known as a converging lens.
Q9. Define the power of a lens. What is its SI unit?
Ans: The power of a lens is the ability to converge or diverge light rays. It is the reciprocal of focal length in metres. Its SI unit is dioptre (D), where 1D = power of a lens with focal length 1 m.
Q10. Why are convex mirrors used as rear-view mirrors in vehicles?
Ans: Convex mirrors provide a wider field of view than plane mirrors, and always form an erect, diminished image of objects behind. This helps drivers see more traffic from a small mirror, ensuring safe driving on roads.
