Two convex lenses of focal length `10 cm` and `20 cm` respectively placed coaxially and are separated by some distance d. The whole system behaves like a concave lens. One of the possible value of d is

A convex lens of focal length 25 cm and a concave lens of focal length 20 cm are mounted coaxially separated by a distance d cm . If the power of the combination is zero, d is equal to

Two plano-convex lenses of focal lengths 20cm and 30cm are placed together to form a double convex lens. The final focal length will be

Two plano-convex lenses of focal lengths 20cm and 30cm are placed together to form a double convex lens. The final focal length will be

A convex lens of focal length 80 cm and a concave lens of focal length 50cm are placed in contact. The resulting will be—

A paralel beam of light incident on a concave lens of focal length 10 cm emerges as a parallel beam from a convex lens placed coaxially, the distance between the lenses being 10 cm. The focal length of the convex lens in cm is

A convex lens and concave lens of focal length +20 cm and -30 cm respectively . Then find the power of each lens. Find the focal length of combined lens. And find the combined power of the lenses (+5D,-3.3D, +60 cm , + 1.7 D)

A convex lens of focal length 100 cm and a concave lens of focal length 10 cm are placed coaxialiy at a separation of 90 cm. If a parallel beam of light is incident on convex lens, then after passing through the two lenses the beam