DAV Class 8 Science Solution Chapter 10 Refraction and Dispersion of Light
DAV School Books Class 8 Science Solution Chapter 10 Refraction and Dispersion of Light all Question Answer. DAV Class 8 10th Chapter Refraction and Dispersion of Light full Chapter explanation by expert teacher.
DAV School Books Class 8 Science Solution Chapter 10 Refraction and Dispersion of Light
A.) Fill in the blanks.
1.) A ray of light, passing from one medium to another, does not bend its path if its angle of incidence equals ___________ degree.
2.) The more is the optical density of a given medium, the ___________ is the speed of light through it.
3.) The phenomenon of the splitting of white light into seven colours is known as the phenomenon of ___________.
4.) The point, on the principal axis of a concave lens, from where a beam of incident parallel ray appears to diverge, is called the ___________ of the concave lens.
5.) An object should be placed at the ___________ point so that a convex lens forms its real and inverted image of the same size.
Answer: (1) 0 (2) less (3) dispersion of light (4) focus (5) 2F
B.) Write True or False for the following statements.
1.) The basic cause of refraction is the change in the speed of light as it goes from one medium to another.
2.) In an optically denser medium, the speed of light is more than the speed of light in vacuum.
3.) An (obliquely) incident ray always bends away from the normal when it passes from one transparent medium into another.
4.) A coin, kept at the bottom of an empty dry cup, appears to rise up’ when some water is poured into the cup.
5.) When white light is ‘dispersed’ by a glass prism, the yellow colour in it, bends more than the blue colour.
Answer: (1) True (2) False, the speed of light in vacuum is the maximum (3) False,, it bends towards the normal (4) True (5) False, it bends lesser than blue colour.
C.) Tick the correct option.
1.) Which of the following conditions is not necessary for a change in the direction of propagation when a light ray goes from one medium to another?
Answer: The incident light rays must always have only the blue colour.
2.) Which of the following diagrams correctly shows the ‘bending of a light ray’ as it goes from an optically denser medium into an optically rarer medium?
Answer: Option B.
3.) The angle, between the incident ray and the emergent ray, for a rectangular glass slab is-
Answer: 0⁰
4.) Which of the following figures correctly represents the passage of white light through a glass prism?
Answer: Option C
5.) An incident ray, passing through the optical centre of a concave lens, after refraction through it, will-
Answer: go undeviated.
6.) When an object approaches a convex lens, from infinity towards its focus, the image, formed by it-
Answer: keeps on shifting away from the lens.
7.) Tanya’s grandmother needs a lens to read small letters in her book. For this she should use-
Answer: a concave lens, kept at a distance less than its focal length from the book.
D.) Answer the following questions in brief.
1.) Light propagates faster through a Medium A than through another Medium B.
(a) Which of the two media has a higher optical density?
Answer: Medium B.
(b) State the condition under which no ‘change in the direction of propagation’ of a light ray would take place when it passes from Medium A to Medium B.
Answer: If the incident ray is perpendicular to the point of incidence.
2.) Why does a pencil appear bent when it is partially dipped in water?
Answer: Light rays from water that enter the air bend in an unusual direction. As a result, light rays seem to be coming from a different location than where the pencil actually is. As a result, when a pencil is partially submerged in water, it appears to be bent.
3.) Trace the path of rays in the following ray diagrams:
4.) When a spherical lens is held towards the Sun and a sharp image is formed on a piece of black paper, a hole gets burnt in the black paper, after some time.
(a) Name the lens used in the above activity.
Answer: Convex lens.
(b) What name is given to the distance between the spherical lens and the black paper?
Answer: Focal length.
(c) What is the relative advantage of using a black paper rather than a white paper?
Answer:
White colour is an excellent reflector thus it reflects more than half of the light energy it receives, whereas black colour is a good absorber of heat and light as a result black cloth gets heated up faster.
5.) What kind of lens would form
(a) an inverted and magnified image of the object?
Answer: Convex lens.
(b) an erect and magnified image of the object?
Answer: Convex lens.
(c) an inverted and diminished image of the object?
Answer: Convex lens.
(d) an erect and diminished image of the object?
Answer: Concave lens.
6.) Draw labelled ray diagrams to illustrate the differences between the virtual images formed by using (a) a concave lens (b) a convex lens.
Answer:
(a) Ray diagrams of the virtual images formed by using a concave lens
(b) Ray diagrams of the virtual images formed by using a convex lens
E.) Answer the following questions.
1.) A coin is placed at the bottom of a clear glass vessel. An observer moves herself to a position where the coin goes just out of sight of her eyes. Keeping her eyes in that position, she asks her friend to pour some water, gradually into the glass vessel.
(a) What would she observe?
Answer: The coins can be seen clearly even after the water is poured into the vessel
(b) Name the phenomenon involved in this observation.
Answer: Refraction of light.
(c) Draw a neat labelled diagram to justify your answer.
Answer:
2.) In the following figures, identify the light ray that would be the correct emergent/refracted light ray in each case. Give reason for your answer in each case.
Answer: In case (a), P is the emergent ray as shown in the diagram it is the ray of light that passes through the glass slab In case (b), R is the emergent ray because as indicated in the diagram it is the light ray that bends towards the normal while it enters the denser medium. In case (c), R is the emergent ray since it is the light ray that bends away from the normal while it enters a rarer medium.
3.) White light is made to fall on one face of a glass prism. Draw a ray diagram to show the nature of the emergent beam.
Answer: White light is bent and divided into the visible spectrum hues of red, orange, yellow, green, blue, and violet when it is made to fall on one face of a glass prism. The prism separates the various colours because it causes their varied wavelengths to bend at angles that are different from one another.
4.) With the help of a suitable diagram, explain the meaning of terms: (a) centres of curvature (b) optical centre (c) focus
Answer:
a) Centers of curvature: The surfaces of two spherical lenses can be considered as parts of sphere the centre of these two spheres are known as centre of curvature.The centres of curvature in this case are C1 and C2..
(b) Optical centre: Optical centre is a point that lies on the principal axis. when a light ray passes through the optical centre it goes straight through it without bending or any kind of deviation from its original path
(c) Focus:When rays of light are parallel to the principal axis of the lens and entered the lens such ray of light either converge at a point on the principal axis or appear to diverge from a point on the principal axis this point is known as principal focus or focus of the lens
5.) We are given a convex lens of focal length 20 cm. Draw ray diagrams to show the nature, size and position of the image formed when the object is kept at a distance of (a) 55 cm (b) 35 cm (c) 15 cm from the lens. (Note: For drawing ray diagrams, use an appropriate scale.)
Answer:
(a)
(b) Diagram below
c)
6.) We are given a concave lens of focal length 15 cm. Draw a ray diagram to show the nature, size and position of the image formed when the object is kept at a distance of (a) 30 cm (b) 15 cm (c) 10 cm from the lens. (Note: For drawing ray diagrams, use an appropriate scale.)
Answer:
(a) 30 cm diagram
(b) 15 cm diagram
(c) 10 cm diagram
7.) What type of a lens is used as a ‘magnifying glass? How is the object positioned with respect to this lens? Draw the appropriate ray diagram.
Answer: Usually, a magnifying glass is essentially a convex lens.Such that the light rays bend and create a magnified image the object has to be placed at a distance larger than the optical centre of the lens.