Class 7 Science Chapter 13 Question Answers - Light

Short Types Questions

Rectilinear propagation of light refers to the movement of light in straight lines in a homogeneous and transparent medium.

Q.2. How are shadows formed?

Shadows are formed when an opaque object obstructs the path of light rays, creating a dark area behind it where the light cannot reach.

Q.3. Define reflection of light.

Reflection of light refers to the bouncing back of light when it strikes the surface of an object and is sent back in the same direction.

Q.4. What is a concave mirror?

A concave mirror is a reflecting surface that curves inward, causing light rays to converge and meet at a point in front of the mirror.

Q.5. What is Newton's disc?

Newton's disc is a circular disc made up of different colors that spin rapidly, producing white light due to the combination of all colors. It demonstrates that white light is made up of a spectrum of colors.

Long Types Questions

Rectilinear propagation of light is the phenomenon where light travels in a straight line in a homogeneous medium. It means that light does not bend or deviate from its path while travelling in a medium. This phenomenon plays a significant role in the formation of shadows. When an opaque object comes in between a source of light and a screen, it blocks the light rays falling on it, creating a dark region behind it. This dark region is called a shadow. The sharp edges of the shadow are due to the rectilinear propagation of light, where the light rays travel in a straight line and do not bend around the edges of the object.

Q.2. Describe the laws of reflection of light.

The laws of reflection of light are as follows:

a) The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

b) The angle of incidence is equal to the angle of reflection, i.e., ∠i = ∠r.

These laws hold true for all types of reflecting surfaces, whether plane or curved. The laws of reflection can be used to predict the path of a reflected ray of light.

Q.3. Explain the working of a concave mirror and its uses.

A concave mirror is a spherical mirror that is curved inward, i.e., its reflecting surface is a part of the inner surface of a sphere. The centre of curvature (C) of the mirror lies behind the mirror surface, and the focus (F) lies in front of it. When a parallel beam of light falls on a concave mirror, the reflected rays converge at the focus. This property of a concave mirror makes it useful in various applications like torches, headlights, and reflecting telescopes. In a torch or a headlight, a concave mirror is used to focus the light rays in a particular direction, making the beam more intense. In a reflecting telescope, a concave mirror is used to collect and focus light from distant celestial objects.

Q.4. Describe the working of a convex lens and its uses.

A convex lens is a curved lens that is thicker at the centre than at the edges. It is also known as a converging lens as it can converge a parallel beam of light to a point called the focus. The distance between the lens and the focus is called the focal length (f). A convex lens can be used in various applications like magnifying glasses, cameras, and projectors. In a magnifying glass, a convex lens is used to magnify the size of an object placed close to it. In a camera, a convex lens is used to focus the light rays on the film, creating a sharp image. In a projector, a convex lens is used to project an enlarged image of a small object onto a screen.

Q.5. Explain the phenomenon of dispersion of light. How is it demonstrated by Newton's Disc?

Dispersion of light is the phenomenon where white light is separated into its constituent colors when it passes through a prism or a lens. This happens because different colors of light have different wavelengths and bend at different angles while passing through a medium. The colors of light are arranged in the order of their increasing wavelengths, forming a spectrum of colors. Newton's disc is a device that demonstrates the phenomenon of dispersion of light. It consists of a disc with different colors arranged in a circular pattern. When the disc is rotated at a high speed, the colors merge to form white light, but as the speed slows down, the colors start to separate, forming a spectrum of colors. The phenomenon of dispersion of light has practical applications in fields like optics and spectroscopy.