A spy satellite orbiting at 160 km above Earth’s surface has a lens with a focal length of 3.6 m and can resolve objects on the ground as small as 30 cm. For example, it can easily measure the size of an aircraft’s air intake port. What is the effective diameter of the lens as determined by diffraction consideration alone? Assume λ=550nm.

Short Answer

Expert verified

The effective diameter of the lens is 0.36 m.

Step by step solution

01

Describe the diffraction by a circular aperture or a lens

The expression to calculate the effective diameter of the lens is given by,

sinθ=1.22λd

Here, λ is the wavelength, d is diameter, and θ is angle.

Let D be the distance between two objects, and L be the distance between Earth and satellite. Then,

Lθ=Dθ=DL

From the above equation,

θ=1.22λdDL=1.22λdd=1.22λLD ….(1)

02

Determine the effective diameter of the lens

Substitute 550×10-9m for λ, 160×103m for L, and 0.30m for D in equation (1).

d=1.22550×10-9m160×103m0.30m=0.36m

Therefore, the effective diameter of the lens is 0.36 m.

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