When a convex lens produces a real image of an object, the minimum distance between the object and image is equal to
- A.the focal length of the convex lens
- B.twice the focal length of the convex lens
- C.four times the focal length of the convex lens
- D.one half of the focal length of the convex lens
Correct answer
Explanation
The correct answer is C, four times the focal length of the convex lens. The gap between an object and its real image is smallest when the object is placed at 2F, twice the focal length from the lens. At that point the image also forms at 2F on the other side, real, inverted and of the same size, so the total distance is 2f + 2f = 4f. If the object is moved closer to F, the image runs away towards infinity; if the object is moved farther than 2F, the image moves in towards F but the object itself is farther off. In both cases the sum of the two distances grows. Using the lens formula, u + v is least when u = v = 2f. Option A is wrong because a distance of only f cannot hold an object and its real image. Option B is wrong because 2f is the distance of each one from the lens, not the distance between them. Option D is wrong because half the focal length is even smaller than f. Exam tip: object at 2F gives an image at 2F of equal size, and the least object-image gap of 4f.
Practice Questions
View allChlorophyll in photosynthetic prokaryotic bacteria is associated with
- A.plastids
- B.membranous vesicles
- C.nucleoids
- D.chromosomes
Show answer
Photosynthetic bacteria have no chloroplasts; their pigments sit on infoldings of the cell membrane that form vesicles or sacs, often called chromatophores.
Read the full explanation →Fat in human body is stored in
- A.Adipose tissue
- B.Red blood cells
- C.Muscle tissue
- D.Epithelial tissue
Show answer
Adipose tissue is a loose connective tissue whose cells, called adipocytes, are built to store fat. Each cell holds a large droplet of fat that pushes the nucleus and cytoplasm to one side.
Read the full explanation →Which one of the following is a metalloid ?
- A.Germanium
- B.Coke
- C.Iodine
- D.Sugar
Show answer
Germanium is a metalloid, an element whose properties lie between those of metals and non-metals. It is a hard, shiny, grey-white solid like a metal, yet it is brittle and conducts electricity far less well than a metal, which makes it a semiconductor.
Read the full explanation →A photon of X-ray has energy of 1 keV. A photon of visible radiation has energy of 3 eV. In this context, which one of the following statements is not correct?
- A.The wavelength of X-ray photon is less than the wavelength of visible radiation photon.
- B.Both the photons have different energies.
- C.The speeds of both the photons in vacuum are different.
- D.The frequency of X-ray photon is higher than the frequency of visible radiation photon.
Show answer
This statement is wrong because every photon, whatever its energy, travels in vacuum at the same speed of light, about 3 × 10^8 metres per second.
Read the full explanation →Which of the following substances cause temporary hardness in water?
1. Mg(HCO3)2
2. Ca(HCO3)2
3. CaCl2
4. MgSO4
Select the correct answer using the code given below.
- A.3 and 4
- B.2 and 3
- C.1 and 4
- D.1 and 2
Show answer
Temporary hardness is caused by the bicarbonates of magnesium and calcium, Mg(HCO3)2 and Ca(HCO3)2. It is called temporary because simple boiling removes it: on heating, the soluble bicarbonates break down into insoluble carbonate or hydroxide, carbon dioxide and water, and the solid settles out.
Read the full explanation →