Questions: Mirrors and lenses
Light: Mirrors and Lenses18 min readLesson 5 of 5
Q1. What is a spherical mirror? How are concave and convex mirrors different?
Answer (English): A spherical mirror is a mirror whose reflecting surface is spherical, shaped like part of a hollow glass sphere. In a concave mirror the reflecting surface curves inward, while in a convex mirror the reflecting surface curves outward. The two mirrors therefore form different kinds of images and are used for different purposes.
Hinglish: Spherical mirror woh mirror hai jiski reflecting surface spherical hoti hai, jaise hollow glass sphere ka koi hissa. Concave mirror mein reflecting surface andar ki taraf curved hoti hai, jabki convex mirror mein bahar ki taraf. Isi wajah se dono mirrors alag tarah ke images banate hain aur alag kaam aate hain.
Q2. Describe the images formed by a concave mirror as the object is moved away from it.
Answer (English): When the object is close to a concave mirror, the image is erect and larger than the object. As the object moves farther away, the image becomes inverted. It is enlarged at first and then keeps getting smaller as the distance increases. The lateral inversion of the image is also seen, as in all mirrors.
Hinglish: Object jab concave mirror ke paas hota hai, image erect aur object se bada hota hai. Object door jaane par image inverted ho jaata hai. Shuru mein image bada hota hai aur phir doori badhne ke saath chhota hota jaata hai. Baaki sab mirrors ki tarah lateral inversion bhi dikhta hai.
Q3. Why are convex mirrors used as side-view mirrors and at road bends?
Answer (English): A convex mirror always forms an erect, diminished image and covers a much wider area than a plane mirror. On a vehicle, this wide view shows more of the traffic behind, though vehicles appear smaller and farther away, which is why the warning about objects being closer is printed. At road bends and intersections, the wide view lets drivers from both sides see the other side and avoid collisions.
Hinglish: Convex mirror hamesha erect, chhota image banata hai aur plane mirror se zyada area dikhata hai. Gaadi par ye wider view peeche ka zyada traffic dikhata hai, halanki vehicles chhote aur door lagte hain, isliye closer-than-they-appear warning likhi hoti hai. Road bends aur intersections par ye wider view dono taraf ke drivers ko doosri side dikhata hai, jisse collisions se bacha jaata hai.
Q4. State the two laws of reflection and explain what happens when light falls along the normal.
Answer (English): The first law states i = r: the reflected angle matches the incident angle, both taken from the normal. The second law says that the incident ray, the reflected ray and the normal all lie in one plane. When light falls along the normal, the angle of incidence is zero, so the angle of reflection is also zero and the light returns along the same path.
Hinglish: Pehla law kehta hai ki angle of incidence, angle of reflection ke barabar hota hai, yaani i = r, aur dono angles normal se naape jaate hain. Doosra law kehta hai ki incident ray, point of incidence par normal, aur reflected ray sab ek hi plane mein hote hain. Jab light normal ke along padti hai, angle of incidence zero hota hai, isliye angle of reflection bhi zero hota hai aur light usi raaste wapas aati hai.
Q5. How does the bent-chart-paper activity prove the second law of reflection?
Answer (English): In the activity, the incident and reflected beams are seen on a chart paper. When the extended part of the paper is bent, the reflected beam disappears from that part. When the paper is flattened again, the beam reappears. That is possible only when the incident ray, the reflected ray and the normal occupy one plane, because bending the paper creates a new plane and the reflected ray no longer lies in it.
Hinglish: Activity mein incident aur reflected beams chart paper par dikhte hain. Paper ka extended hissa modne par reflected beam wahan se gayab ho jaata hai. Paper wapas flat karne par beam phir dikhne lagta hai. Isse pata chalta hai ki incident ray, normal aur reflected ray ek hi plane mein hote hain, kyunki paper modne se naya plane ban jaata hai aur reflected ray usmein nahi rehta.
Q6. What happens when parallel beams fall on a plane mirror, a concave mirror and a convex mirror?
Answer (English): Parallel beams falling on a plane mirror stay parallel after reflection. On a concave mirror, the reflected beams get closer and converge. On a convex mirror, the reflected beams spread apart and diverge. Each ray still obeys the reflection laws, while the curved surface shifts the normal from point to point, so the whole beam converges or diverges.
Hinglish: Plane mirror par parallel beams reflect hone ke baad bhi parallel rehte hain. Concave mirror par reflected beams paas aate hain aur converge karte hain. Convex mirror par reflected beams fail kar diverge karte hain. Har case mein har ray reflection ke laws follow karti hai; curved surface har point par normal ki direction badalta hai, isliye poora beam converge ya diverge hota hai.
Q7. How can a concave mirror burn paper? What is a solar concentrator?
Answer (English): A concave mirror converges the parallel rays of sunlight into a small bright spot. The concentrated light carries energy, and the spot becomes hot enough to scorch or burn paper held there. A device that concentrates sunlight into a small area using mirrors or lenses is called a solar concentrator. Solar cookers and solar furnaces use this idea to cook food or to produce high heat.
Hinglish: Concave mirror suraj ki parallel rays ko chhote bright spot par converge karta hai. Ye concentrated light energy rakhti hai, aur spot itna garam ho jaata hai ki wahan rakha paper jal sakta hai. Jo device mirrors ya lenses se sunlight ko chhote area mein concentrate karta hai use solar concentrator kehte hain. Solar cookers aur solar furnaces isi idea se khana pakate hain ya high heat paate hain.
Q8. How is a convex lens different from a concave lens?
Answer (English): A convex lens is thicker at the middle and thinner at the edges, while a concave lens is thicker at the edges and thinner at the middle. Light is brought together by a convex lens, known as the converging lens, and spread by a concave lens, known as the diverging lens. Images seen through a convex lens can be enlarged, diminished, erect or inverted, but images through a concave lens are always erect and diminished.
Hinglish: Convex lens beech se mota aur kinaron se patla hota hai, jabki concave lens kinaron se mota aur beech se patla. Convex lens light ko converge karta hai isliye converging lens kehlata hai, aur concave lens light ko diverge karta hai isliye diverging lens. Convex lens se image bada, chhota, erect ya inverted ho sakta hai, par concave lens ka image hamesha erect aur chhota hota hai.
Q9. Describe what you see when an object is viewed through a convex lens at different distances.
Answer (English): When the object is close to a convex lens, it appears erect and enlarged. Moving the object farther makes the image turn inverted. It is enlarged at first and then diminishes in size as the distance increases. The exact appearance depends on where the object is placed relative to the lens.
Hinglish: Object jab convex lens ke paas hota hai, woh erect aur bada dikhta hai. Object ko door le jaane par image inverted ho jaata hai. Shuru mein image bada hota hai aur phir doori badhne ke saath chhota hota jaata hai. Exact appearance is baat par depend karti hai ki object lens ke relative kahan rakha hai.
Q10. List four uses of lenses and explain why a pencil looks bent in a glass of water.
Answer (English): Lenses are used in eyeglasses to correct vision, in cameras to focus an image, in telescopes and microscopes to see distant or tiny objects, and in the human eye, whose convex lens changes shape to focus near and far objects. A pencil in a glass of water looks bent at the water surface because light changes direction as it passes from water into air, so the part under water appears shifted from the part above.
Hinglish: Lenses ka use eyeglasses mein vision theek karne ke liye, cameras mein image focus karne ke liye, telescopes aur microscopes mein door ya chhoti cheezein dekhne ke liye, aur hamari aankh mein hota hai, jahan convex lens shape badal kar paas aur door focus karta hai. Paani ke glass mein pencil paani ki surface par muda hua dikhti hai kyunki light paani se hawa mein jaate waqt direction badal leti hai, isliye paani ke andar ka hissa upar ke hisse se shifted lagta hai.
The quick check below has five multiple choice questions on mirrors, lenses and reflection. Each one comes with a short explanation, so take the quiz and see which topics need another read.