Extra 20 important long questions and answers for the "Light" chapter:13 of Class 8 CBSE Science:-
Q: Explain the laws of reflection of light with a neat diagram.
A:
The first law states that the angle of incidence is equal to the angle of reflection.
The second law states that the incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence, all lie in the same plane.
Diagram: A diagram should show a reflecting surface, incident ray, reflected ray, normal, angle of incidence (
i
), and angle of reflection (r), with
i
= r.
Reflection occurs when light bounces back from a smooth surface.
Regular reflection occurs from smooth surfaces, while irregular reflection occurs from rough surfaces.
These laws are fundamental to understanding how mirrors and other reflective surfaces form images.
Q: Describe the formation of images by a plane mirror.
A:
A plane mirror forms a virtual image, which cannot be projected onto a screen.
The image is erect (upright).
The image is of the same size as the object.
The image is laterally inverted (left and right are reversed).
The image is formed as far behind the mirror as the object is in front.
Plane mirrors are very commonly used for everyday life.
Q: Explain the formation of images by a convex mirror and its uses.
A:
A convex mirror always forms a virtual and erect image.
The image is always smaller than the object (diminished).
It provides a wider field of view.
Uses: Rear-view mirrors in vehicles, security mirrors in shops, and street lights.
The wide field of view is due to the outwards curve of the mirror.
The diminished image allows a driver to see a large area behind their vehicle.
Q: Describe the formation of images by a concave mirror and its uses.
A:
A concave mirror can form both real and virtual images, depending on the object's position.
When the object is close, it forms a virtual, erect, and magnified image.
When the object is far, it forms a real and inverted image.
Uses: Headlights of vehicles, shaving mirrors, and by dentists to examine teeth.
Concave mirrors can concentrate light, making them useful in telescopes and solar furnaces.
The magnification property is very helpful in many applications.
Q: Explain the phenomenon of refraction of light with examples.
A:
Refraction is the bending of light as it passes from one transparent medium to another.
It occurs due to the change in the speed of light in different media.
When light passes from a rarer medium to a denser medium, it bends towards the normal.
When light passes from a denser medium to a rarer medium, it bends away from the normal.
Examples: A pencil appearing bent in water, and the formation of a mirage.
Refraction is also the principle behind lenses.
Q: Describe the dispersion of white light and explain the formation of a rainbow.
A:
Dispersion is the splitting of white light into its component
colors
.
It occurs when white light passes through a prism or water droplets.
The different
colors
of light bend at different angles, creating a spectrum.
A rainbow is formed when sunlight is refracted and dispersed by water droplets in the atmosphere.
The
colors
of the rainbow are in the order of VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
Each water droplet acts like a tiny prism.
Q: Explain the structure and functions of the human eye.
A:
Cornea: Transparent front part, refracts light.
Iris:
Colored
part, controls pupil size.
Pupil: Opening in the iris, allows light to enter.
Lens: Convex lens, focuses light on the retina.
Retina: Light-sensitive layer, contains rods and cones.
Optic nerve: Transmits visual information to the brain.
Q: Differentiate between rods and cones in the human eye.
A:
Rods: Sensitive to dim light, responsible for night vision, do not detect
color
.
Cones: Sensitive to bright light, responsible for
color
vision, provide sharp vision.
Rods are more numerous than cones in the retina.
Cones are concentrated in the
center
of the retina (fovea).
Rods provide peripheral vision, while cones provide central vision.
Rods contain the pigment rhodopsin, and cones contain photopsins.
Q: Explain how the human eye adjusts to see objects at different distances.
A:
The lens of the eye changes its shape to focus light on the retina.
For distant objects, the lens becomes thinner.
For nearby objects, the lens becomes thicker.
This adjustment is called accommodation.
The ciliary muscles control the shape of the lens.
This allows the eye to form a sharp image on the retina.
Q: Describe the various defects of vision and their correction.
A:
Myopia (
nearsightedness
): Corrected using concave lenses.
Hyperopia (farsightedness): Corrected using convex lenses.
Presbyopia (age-related farsightedness): Corrected using bifocal lenses.
Astigmatism: Corrected using cylindrical lenses.
Cataract: Corrected by surgical removal of the cloudy lens.
Early detection and correction are important for maintaining good vision.
Q: Explain the concept of lateral inversion in a plane mirror.
A:
Lateral inversion is the reversal of left and right in the image formed by a plane mirror.
The image appears as if it has been flipped horizontally.
If you raise your right hand, the image appears to raise its left hand.
This is why an ambulance is written backwards on the front, so it appears correctly in a
rear view
mirror.
It occurs because the mirror reverses the depth of the image.
It is a characteristic property of plane mirror images.
Q: Describe the applications of optical
fibers
.
A:
Communication: Transmitting data over long distances using light signals.
Medical imaging: Endoscopes use optical
fibers
to view internal organs.
Sensors: Detecting changes in temperature, pressure, and other parameters.
Lighting: Decorative lighting, and illumination in hard to reach places.
Industrial inspection: Viewing inside machinery, and pipes.
Optical
fibers
use the principle of total internal reflection.
Q: Explain the difference between regular and irregular reflection.
A:
Regular reflection: Occurs from smooth surfaces, like mirrors.
Irregular reflection: Occurs from rough surfaces, like paper or walls.
In regular reflection, parallel incident rays are reflected as parallel rays.
In irregular reflection, parallel incident rays are reflected in different directions.
Regular reflection forms clear images, while irregular reflection does not.
The smoothness of the reflecting surface determines the type of reflection.
Q: Describe the formation of a virtual image by a concave mirror when the object is between the pole and focus.
A:
When an object is placed between the pole and focus of a concave mirror, a virtual image is formed.
The image is formed behind the mirror.
The image is erect.
The image is magnified.
This principle is used in shaving mirrors and dental mirrors.
The reflected rays appear to diverge from a point behind the mirror.
Q: Explain how the intensity of light affects the human eye's pupil size.
A:
In bright light, the pupil contracts to reduce the amount of light entering the eye.
In dim light, the pupil dilates to increase the amount of light entering the eye.
The iris controls the size of the pupil.
This adjustment helps protect the retina from damage in bright light.
It also helps improve vision in low light conditions.
The pupil acts like an aperture in a camera.
Q: Describe the use of concave mirrors in solar furnaces.
A:
Concave mirrors can concentrate sunlight at their focal point.
In solar furnaces, large concave mirrors are used to focus sunlight onto a small area.
This concentration of sunlight generates high temperatures.
These high temperatures can be used to melt metals or produce steam.
Solar furnaces provide a clean and renewable energy source.
They are used in industrial applications for high-temperature processes.
Q: Explain the formation of images in a kaleidoscope.
A:
A kaleidoscope uses multiple mirrors to create patterns.
The mirrors are arranged at specific angles, usually 45 or 60 degrees.
Small objects inside the kaleidoscope are reflected multiple times.
This creates symmetrical and
colorful
patterns.
The patterns change as the kaleidoscope is rotated.
The multiple reflections are due to the laws of reflection.
Q: Describe the use of convex mirrors in streetlights.
A:
Convex mirrors are used in streetlights to spread light over a wide area.
The diverging nature of convex mirrors helps
illuminate
a large space.
This improves visibility and safety on roads and pathways.
Convex mirrors provide a wide field of view.
They are mounted at a height to maximize the coverage.
They help reduce blind spots.
Q: Explain the concept of total internal reflection and its applications.
A:
Total internal reflection occurs when light travels from a denser medium to a rarer medium at an angle greater than the critical angle.
All the light is reflected back into the denser medium.
Applications include optical
fibers
, prisms, and diamond cutting.
Optical
fibers
use total internal reflection to transmit light signals over long distances.
Prisms use total internal reflection to invert or deviate light without loss of intensity.
This principle is used in many optical devices.
Q: Describe the importance of the sense of sight and how to care for our eyes.
A:
Sight helps us perceive the world around us.
It allows us to read, write, and recognize objects and people.
To care for our eyes, we should avoid rubbing them and wash them with clean water.
We should avoid looking directly at the sun or bright lights.
We should maintain a proper distance while reading or watching television.
Regular eye
checkups
are important to detect and correct vision problems.
