Lenses
Chapter 7: Lenses
- An object kept 60 cm from a lens gives a virtual image 20 cm in front of the lens. Answer: focal length = −30 cm; it is a diverging (concave) lens. Using the Cartesian sign convention, the object is on the left: u = -60 cm.... 1
- Three lenses having power 2, 2.5 and 1.7 D are kept touching in a row. What is the total power of the lens combination? Answer: 6.2 D When thin lenses are kept in contact, the power of the combination is the sum of their individual powers: P = P₁ + P₂ + P₃ = 2... 1
- 5 cm high object is placed at a distance of 25 cm from a converging lens of focal length of 10 cm. Answer: the image is 16.7 cm behind the lens, 3.33 cm tall, real, inverted and diminished. Using the Cartesian sign convention: u = -25 cm,... 1
- Doctor has prescribed a lens having power +1.5 D. What will be the focal length of the lens? What is the type of the lens and what must be the defect of vision? Given: Power of lens, P = +1.5 D, f=? Now, focal length of lens, f=1/P =1/(+1.5D) =10/15 m =+0.67 m P is positive, This shows the lens is co... 1
- What is the function of iris and the muscles connected to the lens in human eye? Function of Iris: The iris is a muscular diaphragm that controls the size of the pupil, which, in turn, controls the amount of light enterin... 1
- Distinguish between b. Concave lens and Convex Lens Concave lens Convex lens It is a diverging lens because it diverges the rays falling on it. It is a converging lens because it converges the... 1
- Distinguish between: a. Farsightedness and Nearsightedness Farsightedness (hypermetropia) Nearsightedness (Myopia) In hypermetropia, person clearly sees all the far away objects, but is unable to see... 1
- Explain the working of an astronomical telescope using refraction of light. The astronomical telescope consists of two lenses: objective and eyepiece. Objective has larger focal length and diameter to accommodate max... 1
- We can not clearly see an object kept at a distance less than 25 cm from the eye. We cannot clearly see an object kept at a distance less than 25 cm from the eye. This is because when we try to see a nearby object, the eye... 1
- One can sense colours only in bright light. The retina in the eye is made of many light sensitive cells. The rod-shaped cells respond to the intensity of light while the cone-shaped ce... 1
- Simple microscope is used for watch repairs. When an object is placed within the focal length of a magnifying glass or simple microscope (convex lens), its larger and erect image is obt... 1
- At which position will you keep an object in front of a convex lens so as to get a real image of the same size as the object ? Draw a figure. When an object is placed at the centre of curvature 2F₁ of a convex lens, we will get a real image 2F₂ of the same size as the object. 1
- Draw a figure explaining various terms related to a lens Various Terms related with Lenses: 1) Optical centre : The point inside a lens on the principal axis, through which light rays pass without... 1
- Q1} Match the columns in the following table and explain them Column 1 Column 2 Column 3 Farsightedness Faraway object can be seen clearly Convex lens Presbyopia Problem of old age Bifocal lens Nearsigh... 2