NCERT 7th Class (CBSE) Science: Light

Question: How will you distinguish between a plane mirror, and a convex mirror without touching them?

Answer: We can distinguish between these mirror just by bringing our face close to each mirror, turn by turn. A plane mirror will produce an image of the same size as our face and we will look our normal self. A concave mirror will produce a magnified image and our face will look much bigger. A convex mirror will look much smaller like a small child.

Question: How will you show that a convex lens converges parallel rays of light to a single point?

Answer: Place a piece of paper on the ground in bright sun light. Hold a convex lens some distance above the pieces of paper in such a way that a sharp image of the of the sun is formed on the piece of paper. Here the convex lens is converging the parallel rays of sunlight due to which the sun’s rays get concentrated on a small part of the paper. The heat energy of focused sunlight rays burns the piece of paper.

Question: Name type of mirror used by dentists. Why is it used for this purpose?

Answer: Concave mirrors are used by dentists to see the large images of teeth of patients. Due to this, it becomes easier to locate the defect in the teeth.

Question: Name the type of mirror which is used as a reflector inn the head lights of a car. Why is it used for this purpose?

Answer: Concave mirror is used as a reflector in the head lights of a car. When a lighted bulb is placed at the focus of a concave reflector, then the concave reflector collects all the diverging rays of light of the bulb and converts them into a power beam of parallel rays. This parallel beam of light rays can reach a large distance in the darkness of night and make us see things up to considerable distances.

Question: Define dispersion of white light and name the colours of white light in order.

Answer: The splitting of while light into its constituent seven colours is called dispersion of white light. White light splits into the following colours: violet, indigo, blue, green, yellow, orange and red.