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Q1. Why is a concave mirror used by dentists to examine teeth?
Dentists use concave mirrors because when the tooth is placed between the focus and the pole, the mirror forms a virtual, erect, and magnified (enlarged) image. This helps dentists see small cavities and details clearly for proper treatment.
Q2. Why are convex mirrors used as rear-view mirrors in cars and scooters?
Convex mirrors provide a wider field of view because they bulge outward. Although they make objects appear smaller and farther away, they allow the driver to see a larger area behind the vehicle, which is essential for safe driving.
Q3. Why is a concave reflector used in a torch?
In a torch, the bulb is placed at the focus of a concave mirror. The light rays from the bulb reflect off the concave mirror and emerge as a parallel beam. This directs light in a specific direction over a long distance.
Q4. Why do our feet look shorter when standing in clear water?
When we stand in water, light rays from our feet travel from water (denser medium) to air (rarer medium). They bend away from the normal at the water surface. Our brain interprets these bent rays as coming from a shallower position, making the feet appear shorter.
Q5. Why does a pencil appear bent when dipped obliquely in a glass of water?
When light from the submerged part of the pencil travels from water to air, it bends (refracts) at the water surface. Our brain assumes light travels in a straight line, so the pencil appears bent or broken at the water surface.
Q6. Why are concave mirrors used in solar cookers?
Concave mirrors converge parallel rays of sunlight to a point called the focus. This concentration of light produces intense heat, which is used for cooking food in solar cookers.
Q7. Why is it difficult to catch a fish in water with a spear by aiming directly at it?
Light from the fish travels from water to air and bends (refracts) at the surface. This makes the fish appear at a different position (shallower) than its actual location. So, aiming directly at the apparent image causes the spear to miss the actual fish.
Q8. Why are reading glasses (for old age) usually convex lenses?
With age, the eye’s lens loses its ability to converge light sufficiently for nearby objects (hypermetropia/far-sightedness). Convex lenses converge light before it enters the eye, helping to focus the image on the retina for clear near vision.
Q9. Why are concave lenses used in spectacles for short-sighted people (myopia)?
In myopia, the eye converges light too much, forming the image in front of the retina. Concave lenses diverge the light rays slightly before they enter the eye, moving the focus back onto the retina so distant objects appear clear.
Q10. Why do stars twinkle?
Starlight passes through different layers of Earth’s atmosphere with varying refractive indices. This causes the light to bend continuously, and the amount of light reaching our eyes fluctuates. This makes the stars appear to twinkle.
Q11. Why does the Sun appear flattened at sunrise and sunset?
At sunrise and sunset, sunlight passes through a thicker layer of the atmosphere. The lower part of the Sun is refracted more than the upper part, causing it to appear flattened or oval-shaped. This is called atmospheric refraction.
Q12. Why are convex mirrors used at blind turns on roads?
Convex mirrors give a wider field of view. At blind turns, they allow drivers to see vehicles or pedestrians coming from the other side, helping to prevent accidents.
Q13. Why is a convex lens used in a simple camera?
In a camera, the convex lens converges light rays from the object and forms a real, inverted, and diminished image on the film or sensor. This image is then captured to create the photograph.
Q14. Why does a pool of water appear shallower than it actually is?
Light from the bottom of the pool travels from water to air and bends away from the normal. Our brain interprets these bent rays as coming from a shallower position, so the pool appears less deep than it actually is.
Q15. Why are concave mirrors used in headlights of cars and searchlights?
The bulb is placed at the focus of the concave mirror. The light rays reflect off the concave surface and emerge as a parallel beam, producing a strong, focused beam that illuminates the road ahead effectively.
Q16. Why does a lemon in a glass of water appear bigger than its actual size?
The curved glass filled with water acts like a convex lens. When light from the lemon passes through the water and glass, it refracts and bends, making the lemon appear larger than its actual size.
Q17. Why is it advised to use a concave mirror for shaving?
When the face is held close to a concave mirror (between the focus and the pole), the mirror forms a virtual, erect, and magnified image. This enlargement helps in seeing small details clearly, making shaving easier.
Q18. Why does a driver see a rainy road at night as a dark, rough surface instead of a mirror?
A rainy road surface is rough and uneven. Light from the headlights reflects in many different directions (diffuse reflection) rather than in one direction. This scatters the light, so the road appears dark and rough instead of like a shiny mirror.
Q19. Why do diamonds sparkle?
Diamond has a very high refractive index (n ≈ 2.42). When cut properly, light entering the diamond undergoes multiple total internal reflections before emerging. This causes the diamond to sparkle brilliantly.
Q20. Why is a magnifying glass (a convex lens) used to read small print?
When a convex lens is placed close to the page with the letters between the lens and its focus, it forms a virtual, erect, and magnified image. This enlarges the small print, making it easier to read.
Q21. Why does a coin at the bottom of a bucket filled with water appear raised?
Light from the coin travels from water to air and bends at the surface. Our brain assumes the light has traveled in a straight line, so the coin appears to be at a higher (raised) position than its actual depth.
Q22. Why are convex mirrors used in supermarkets and stores for security?
Convex mirrors provide a wider field of view. A single convex mirror placed in a corner can show a large part of the store, helping staff monitor customers and prevent shoplifting.
Q23. Why does a stick partially immersed in water look broken at the water surface?
Light from the submerged part of the stick bends as it travels from water to air. This bending (refraction) makes the stick appear broken or bent at the water surface because our brain expects light to travel in straight lines.
Q24. Why are concave mirrors not used as rear-view mirrors in vehicles?
Concave mirrors have a limited field of view. Also, they form inverted images when the object is beyond the focus, which would be confusing for a driver. Convex mirrors are preferred because they provide a wider, erect view.
Q25. Why do advanced telescopes use large concave mirrors instead of lenses?
Lenses bend different colors of light by different amounts (chromatic aberration), causing color fringing. Mirrors reflect all colors equally and do not have this problem. Also, large mirrors can be made lighter and more stable than large lenses.
Q26. Why does a person with hypermetropia (far-sightedness) use convex lens spectacles?
In hypermetropia, the eye cannot focus nearby objects clearly because the image forms behind the retina. Convex lenses converge the light rays more, helping to focus the image on the retina for clear near vision.
Q27. Why does a glass slab placed over writing not magnify it, but a convex lens does?
A glass slab has two parallel surfaces. Light entering and leaving the slab refracts twice but emerges parallel to the incident ray, causing only a slight shift, not magnification. A convex lens has curved surfaces that converge light to form a magnified image.
Q28. Why do we sometimes see a mirage of water on a hot road?
On a hot day, the air near the road is hotter and less dense than the air above. Light from the sky undergoes total internal reflection at the boundary between the hot and cooler air. This reflected light appears as a shiny patch resembling water.
Q29. Why is the power of a doctor’s reading glasses expressed in positive dioptres?
Reading glasses for old age correct hypermetropia (far-sightedness) using convex lenses. Convex lenses have positive focal lengths, so their power (P = 1/f) is positive. The power is measured in positive dioptres.
Q30. Why do objects under water look closer and bigger when viewed from above?
Light from underwater objects bends when it enters the air. Our brain assumes light travels in straight lines, so it projects the image to a shallower position. This makes objects appear closer and slightly larger than they actually are.
Q31. Why are side mirrors of cars labeled “Objects in mirror are closer than they appear”?
Convex mirrors form diminished images, making objects appear smaller and therefore farther away than they actually are. The warning reminds drivers that the vehicle behind is actually closer than it appears in the mirror.
Q32. Why does a rainbow appear as an arc?
Sunlight enters raindrops, disperses into different colors, undergoes total internal reflection inside the drop, and then refracts out. Raindrops at a specific angle (about 42°) produce the rainbow. The collective effect of millions of drops forms an arc in the sky.
Q33. Why does a person’s legs look short when they are standing in a swimming pool?
Light from the submerged legs travels from water (denser) to air (rarer) and bends away from the normal. Our brain interprets these bent rays as coming from a shallower position, making the legs appear shorter than they actually are.
Q34. Why is a concave mirror suitable for applying makeup?
When the face is held between the focus and the pole of a concave mirror, it forms a virtual, erect, and magnified image. This enlargement helps in seeing small details like skin pores or eyebrows, making makeup application easier.
Q35. Why does a thick glass window sometimes make objects viewed through it look slightly shifted?
When light passes through a glass window, it refracts twice—once when entering and once when leaving the glass. This causes a slight lateral shift in the position of objects, making them appear slightly displaced from their actual position.
Q36. Why are concave mirrors used in satellite TV dishes?
Satellite signals are very weak when they reach Earth. The concave dish reflects and converges these signals to a point (the focus) where the receiver is placed. This concentrates the signal, making it strong enough to be detected.
Q37. Why does a drop of water on a printed page act as a magnifier?
A small drop of water has a curved surface. This curved surface acts like a convex lens. When placed over print, it magnifies the letters, making them appear larger and clearer.
Q38. Why do spectacles for myopia have lenses with negative power?
Myopia (short-sightedness) is corrected using concave lenses. Concave lenses have negative focal lengths, so their power (P = 1/f) is negative. The power is expressed in negative dioptres.
Q39. Why does the setting Sun appear red?
At sunset, sunlight travels through a thicker layer of the atmosphere. Blue light is scattered away, while red light, which scatters less, passes through and reaches our eyes. This makes the Sun appear reddish.
Q40. Why is it easier to read a book under a lamp that has an opaque shade directing light downwards?
An opaque shade directs light directly onto the page, increasing illumination. It also prevents direct light from entering the eyes, reducing glare. The page reflects light diffusely, making reading more comfortable.
Q41. Why does a convex lens held away from a paper under the Sun can burn a hole in it?
A convex lens converges parallel rays of sunlight to a point called the focus. The concentrated light produces intense heat at the focus, which can burn a hole in paper. This demonstrates the focusing power of convex lenses.
Q42. Why does a person wearing spectacles sometimes see a glaring circle of light around car headlights at night?
Light from headlights reflects and refracts multiple times on the curved surfaces of spectacle lenses. This causes internal reflections within the lens, creating a glaring circle around the light source.
Q43. Why are overhead projectors and slide projectors equipped with convex lenses?
In projectors, the slide or transparency is placed between the focus and the lens. The convex lens forms a real, inverted, and enlarged image on the screen, making the content visible to a large audience.
Q44. Why does a white shirt look brighter than a black shirt under the same light?
A white shirt reflects almost all light that falls on it, so it appears bright. A black shirt absorbs most light and reflects very little, so it appears dark. This is why black clothes feel warmer in sunlight.
Q45. Why is a periscope in a submarine uses plane mirrors and not lenses?
Plane mirrors in a periscope reflect light without dispersing colors (no chromatic aberration) and do not magnify or distort the image. Lenses would introduce color fringing and could magnify or invert the image, which is not desirable for navigation.
Q46. Why does a fluorescent tube light seem to extend beyond its physical ends when viewed from certain angles?
Light from inside the fluorescent tube travels through the glass and undergoes total internal reflection at the ends. Some light is trapped and emerges from the sides, making the tube appear to glow beyond its physical ends.
Q47. Why is a peephole (door viewer) in a door made with a combination of lenses?
A peephole uses a combination of lenses (usually a concave lens and a convex lens) to give a wide-angle, diminished image of the outside. This allows the person inside to see who is at the door without opening it.
Q48. Why does a wet, black asphalt road look darker than a dry one?
Dry asphalt has a rough surface that scatters light in many directions (diffuse reflection), making it appear lighter. When wet, water fills the gaps, creating a smoother surface. This reduces scattering and causes more light to be absorbed, making the road appear darker.
Q49. Why is it hard to judge the distance of an oncoming car at night if it has only one headlight working?
Our brain uses binocular vision and the distance between two points (like headlights) to estimate distance. With only one headlight, there is no separation to judge depth, making it difficult to accurately determine how far away the car is.
Q50. Why does a convex lens used as a magnifying glass not work well if the object is too far from the lens?
A magnifying glass works by forming a virtual, erect, and magnified image when the object is between the lens and its focus. If the object is too far (beyond the focus), the lens forms a real and inverted image, which is not useful for magnification.