Light – Case-based Questions with Answers
CBSE Class 8 Science – Chapter 16: Light (Case-Based Questions)
CBSE Board Examinations – Topic-wise Case-Based Questions with Answers
Light, Visibility and Rectilinear Propagation
One evening, there is a sudden power cut in Riya’s house. The room becomes completely dark and she cannot see any object around her. After some time, her mother lights a candle. Immediately, Riya can see the furniture, books and other objects in the room again.
(a) Why was Riya unable to see anything during the power cut?
(b) How did lighting a candle make the objects visible again?
(c) Name the type of object a candle flame is: luminous or non-luminous?
(a) Answer: During the power cut there was no light in the room, so no light could enter Riya’s eyes from the objects. Hence she could not see them.
(b) Answer: The candle flame produced light which fell on the objects and got reflected into Riya’s eyes, making the objects visible again.
(c) Answer: A candle flame is a luminous object because it gives out its own light.
In a school activity, the teacher gives three cardboards with tiny holes at their centres to a group of students. The students arrange the cardboards in a straight line and place a burning candle in front of the first cardboard. When they look through the holes, they can see the candle flame. If they slightly shift one cardboard sideways, the candle flame is no longer visible.
(a) What property of light does this activity show?
(b) Why is the candle flame not seen when one cardboard is shifted?
(c) Mention any one more example that shows the same property of light.
(a) Answer: The activity shows the rectilinear propagation of light, i.e. light travels in a straight line.
(b) Answer: When one cardboard is shifted, the three holes are no longer in the same straight line, so the straight path of light from the candle to the eye is blocked and the flame is not seen.
(c) Answer: Formation of sharp shadows of objects in sunlight is another example showing that light travels in straight lines.
During a science period, the teacher shows three objects to the class: a wooden block, a frosted glass sheet and a clear glass plate. She places each object in the path of light and asks students to observe the shadow on a screen placed behind the objects.
(a) Which object will form a dark and sharp shadow? Why?
(b) What kind of shadow will a frosted glass sheet form?
(c) Which type of material does not form a clear shadow at all?
(a) Answer: The wooden block will form a dark and sharp shadow because it is an opaque object and completely blocks the light.
(b) Answer: The frosted glass sheet will form a faint or blurred shadow because it is translucent and allows some light to pass through.
(c) Answer: A transparent material like clear glass does not form a clear shadow because it allows most of the light to pass through.
On a sunny day, Amit observes the shadow of a tree at different times. In the early morning, the shadow is very long. At noon, the shadow becomes very short. In the evening, the shadow again becomes long but in the opposite direction.
(a) Why is the shadow long in the morning and evening?
(b) Why does the shadow become short at noon?
(c) What does this observation tell us about the path of sunlight?
(a) Answer: In the morning and evening, the Sun is low in the sky, so its rays fall obliquely on the objects, producing long shadows.
(b) Answer: At noon, the Sun is almost overhead and its rays fall nearly vertically, so the shadow is short.
(c) Answer: It shows that sunlight travels in straight lines and the direction of light affects the length and direction of shadows.
Reflection, Plane Mirrors and Lateral Inversion
Seema shines the beam of a torch on a plane mirror kept on a table. She notices that the light falling on the mirror is sent back in a definite direction. By moving the torch, she observes that the direction of the reflected beam also changes in a regular way.
(a) What phenomenon of light is being observed here?
(b) Name the rays and angle involved when light falls on a mirror.
(c) State the relationship between angle of incidence and angle of reflection in this case.
(a) Answer: The phenomenon is reflection of light from a plane mirror.
(b) Answer: The ray falling on the mirror is the incident ray, the ray leaving the mirror is the reflected ray, and the angle between incident ray and normal is angle of incidence; between reflected ray and normal is angle of reflection.
(c) Answer: The angle of reflection is always equal to the angle of incidence (∠r = ∠i).
Rohan notices that the word “AMBULANCE” is written in a reversed manner on the front of an ambulance. When drivers in front look into their rear-view mirror, they can read the word “AMBULANCE” correctly and give way quickly.
(a) What effect of a plane mirror is being used in this case?
(b) What do we mean by lateral inversion?
(c) Why is this mirror effect useful for ambulances?
(a) Answer: The effect used here is lateral inversion produced by a plane mirror.
(b) Answer: Lateral inversion is the left-right reversal of an image in a plane mirror, where the left side appears right and right side appears left.
(c) Answer: Because of lateral inversion, the reversed word on the ambulance appears correct in the rear-view mirror, helping other drivers recognise it quickly and allow it to pass.
In the classroom, a tube light is switched on. Neha can see the tube light clearly in the plane mirror on the wall, but she cannot see its clear image on the painted wall itself. The wall appears bright, but no sharp image is formed on it.
(a) What type of reflection does the plane mirror give?
(b) What type of reflection does the painted wall give?
(c) Why is a clear image formed only in the mirror and not on the wall?
(a) Answer: The plane mirror gives regular reflection.
(b) Answer: The painted wall gives diffused (irregular) reflection.
(c) Answer: A smooth mirror reflects parallel rays in a regular way, forming a clear image. A rough wall scatters the rays in many directions, so no sharp image is formed.
A girl stands 1.5 m in front of a large plane mirror in her house. She notices that her image appears to be as far behind the mirror as she is in front of it. Her image is upright and of the same size as her body.
(a) What is the distance between the girl and her image?
(b) Name two characteristics of the image formed by the plane mirror in this case.
(c) Can this image be obtained on a screen? Why or why not?
(a) Answer: The image is 1.5 m behind the mirror, so the distance between the girl and her image is 1.5 m + 1.5 m = 3.0 m.
(b) Answer: The image is virtual and erect, and it is of the same size as the girl.
(c) Answer: No, it cannot be obtained on a screen because it is a virtual image formed by the apparent meeting of reflected rays behind the mirror.
Multiple Reflection, Kaleidoscope and Periscope
In an activity, two plane mirrors are placed at an angle of 60° to each other. A small toy is kept between them. When Ankit looks into one mirror, he can see several images of the toy. When the angle between the mirrors is reduced, the number of images increases.
(a) Why are multiple images of the toy seen in the mirrors?
(b) What happens to the number of images when the angle between the mirrors decreases?
(c) Name one device that uses the principle of multiple reflection.
(a) Answer: Light from the toy reflects back and forth between the two mirrors many times, producing multiple images due to repeated reflection.
(b) Answer: When the angle between the mirrors decreases, the number of images increases.
(c) Answer: A kaleidoscope uses the principle of multiple reflection.
Megha makes a simple kaleidoscope using three plane mirrors arranged in the form of a triangular tube. She puts small coloured glass pieces at one end and looks through the other end. On rotating the kaleidoscope, she sees changing colourful patterns.
(a) Which principle of light does a kaleidoscope use?
(b) Why do the patterns change when Megha rotates the kaleidoscope?
(c) Mention one use of a kaleidoscope in real life.
(a) Answer: A kaleidoscope uses multiple reflection of light from its mirrors.
(b) Answer: When she rotates the kaleidoscope, the positions of the coloured glass pieces change, so their reflections in the mirrors also change, producing new patterns.
(c) Answer: Designers use kaleidoscopes to get ideas for patterns on fabrics, wallpapers and decorative items.
A group of students makes a simple periscope using a long cardboard tube and two plane mirrors placed at 45° at the two ends. When one student looks through the lower end, he can see objects placed above a wall which otherwise hide them from view.
(a) What is the function of a periscope?
(b) How many plane mirrors are used in this simple periscope?
(c) Name one place where such an instrument is very useful.
(a) Answer: A periscope helps a person see objects that are not directly in the line of sight, such as over a wall or from inside a trench.
(b) Answer: Two plane mirrors are used in the simple periscope.
(c) Answer: Periscopes are very useful in submarines for viewing objects above the water surface.
In a decoration shop, Ritu notices an arrangement where a row of small bulbs is placed between two facing mirrors. When the bulbs are switched on, it appears as if there is a never-ending tunnel of lights in the mirrors.
(a) Why does Ritu see so many images of the same row of bulbs?
(b) What happens to the brightness of the images as they go deeper into the “tunnel”?
(c) Which property of light is again responsible for this effect?
(a) Answer: The two mirrors reflect light back and forth repeatedly, forming many images of the same row of bulbs due to multiple reflection.
(b) Answer: The brightness of the images decreases as they go deeper because some light is absorbed or lost at each reflection.
(c) Answer: The effect is due to multiple reflection of light between two plane mirrors.
Sunlight, Dispersion, Spectrum and Rainbow
In a darkened room, a physics teacher allows a thin beam of sunlight to pass through a small hole in the window. The beam is then allowed to fall on a glass prism. On the other side of the prism, a band of different colours appears on a white screen.
(a) What is the band of colours obtained on the screen called?
(b) Name the phenomenon responsible for the formation of this band.
(c) What does this experiment prove about white sunlight?
(a) Answer: The band of colours is called a spectrum.
(b) Answer: The phenomenon is called dispersion of light.
(c) Answer: It proves that white sunlight is actually a mixture of many colours.
During the prism experiment, the students observe that the spectrum always appears with violet at one end and red at the other end. The teacher tells them to remember the order of colours with the word “VIBGYOR”.
(a) Expand VIBGYOR.
(b) Which colour bends the most in the prism?
(c) Which colour bends the least in the prism?
(a) Answer: VIBGYOR stands for Violet, Indigo, Blue, Green, Yellow, Orange and Red.
(b) Answer: Violet light bends the most in the prism.
(c) Answer: Red light bends the least in the prism.
After a sudden shower on a sunny afternoon, Rakesh sees a beautiful rainbow in the sky. The Sun is behind him and the rainbow appears in the sky in front of him, opposite to the Sun. He notices that the rainbow has the same colour order as the prism spectrum.
(a) What natural phenomenon is Rakesh observing?
(b) What acts like tiny prisms in the formation of a rainbow?
(c) Why is a rainbow always seen in a direction opposite to the Sun?
(a) Answer: He is observing a rainbow, a natural spectrum.
(b) Answer: Water droplets in the atmosphere act like tiny prisms.
(c) Answer: Because sunlight enters the droplets opposite to the Sun and then reaches our eyes, we see the rainbow in the portion of the sky opposite to the Sun.
In the laboratory, a student passes a beam of white light through a glass prism and gets a coloured spectrum. Later, she passes the same beam through a rectangular glass slab but does not observe any spectrum.
(a) Why does the prism produce a spectrum?
(b) Why does the glass slab not produce a separate spectrum?
(c) What conclusion can you draw about the shapes of these two glass objects?
(a) Answer: The prism has non-parallel refracting surfaces, so different colours of light bend by different amounts and spread out to form a spectrum.
(b) Answer: In a glass slab, opposite faces are parallel. The colours that spread while entering combine again while leaving, so no separate spectrum is seen.
(c) Answer: The non-parallel faces of a prism cause dispersion, while the parallel faces of a rectangular slab do not show dispersion clearly.
Human Eye, Eye Care and Braille System
Rahul likes to read comics at night in dim light, even after his parents tell him to switch on a proper lamp. After a few days, he complains of headache and difficulty in seeing the writing on the blackboard clearly from the last bench in class.
(a) Which sense organ is being affected by Rahul’s habit?
(b) Why can reading in dim light be harmful for the eyes?
(c) What should Rahul do now to take care of his eyes?
(a) Answer: His eyes are being affected.
(b) Answer: In dim light, the eyes have to strain more to see clearly, which can cause headaches, tiredness and may lead to vision problems.
(c) Answer: He should read only in proper light, maintain a correct distance from books and consult an eye specialist if the problem continues.
During a lesson, the teacher shows a diagram of the human eye and explains that light enters through the cornea and pupil, is focused by the eye lens and finally forms an image on the retina. The optic nerve carries the information to the brain.
(a) Which part of the eye controls the amount of light entering the eye?
(b) On which part of the eye is the image formed?
(c) Which part of the body helps us to actually “see” the image formed by the eye?
(a) Answer: The iris controls the size of the pupil and thus the amount of light entering the eye.
(b) Answer: The image is formed on the retina.
(c) Answer: The brain interprets the signals from the retina and helps us actually “see” the object.
Suresh visits a school for visually impaired children. He notices that students are reading books by moving their fingers over pages with raised dots instead of printed letters. The teacher tells him that this system is called Braille.
(a) What is the Braille system?
(b) How do visually impaired students read using Braille?
(c) Name the person who developed the Braille system.
(a) Answer: The Braille system is a method of reading and writing for visually impaired persons using patterns of raised dots.
(b) Answer: They move their fingertips over the raised dots, feel the patterns and recognise letters and numbers through touch.
(c) Answer: The Braille system was developed by Louis Braille.
During a solar eclipse, some children try to look directly at the Sun with their naked eyes. The science teacher quickly stops them and explains that doing so may damage their eyesight permanently. She also advises them about safe methods to view such events.
(a) Why is it dangerous to look directly at the Sun or a solar eclipse?
(b) Suggest one safe way to observe a solar eclipse.
(c) Mention any one general precaution to protect our eyes in daily life.
(a) Answer: The intense light and heat from the Sun can damage the delicate retina and may cause permanent loss of vision.
(b) Answer: One safe way is to use special eclipse glasses or to view the eclipse indirectly by a pinhole projection method.
(c) Answer: We should avoid reading in very dim light or sitting too close to TV/mobile screens to protect our eyes.