Patent ID: 8373930

Claim:
An optical element comprising: a base member; and a first layer which is formed on the base member and whose refractive index for a central use wavelength λ changes in a direction of a thickness of the first layer by 0.05 or more, wherein the first layer has an anti-reflection function and satisfies the following conditions: n t = n i + 0.1 · ( n s - n i ) 0.5 ≤ n ⁢ { t ⁡ ( n t ) / 2 } - n t n s - n ⁢ { t ⁡ ( n t ) / 2 } ≤ 0.8 λ 4 ≤ t ⁡ ( n t ) ≤ 2 ⁢ λ 1.0 ≤ n i ≤ 1.1 where n i represents a refractive index of a most light entrance side part of the first layer for the central use wavelength, n s represents a refractive index of a most base member side part of the first layer for the central use wavelength, t(n t ) represents an optical film thickness of the first layer at which the refractive index thereof for the central use wavelength is n t , and n{t(n t )/2} represents a refractive index of the first layer for the central use wavelength at a position where the optical film thickness is t(n t )/2.