Optical element, optical system and optical apparatus having antireflection coating

An optical system comprising an optical element that includes a substrate that is transparent to light having a used wavelength region and an antireflection coating formed on the substrate,

• wherein the antireflection coating includes an intermediate layer formed on the substrate, and an undulation layer that is formed on the intermediate layer and has a plurality of convexes arrayed with spacing shorter than a shortest wavelength in the used wavelength region,
• wherein the undulation layer has a portion where refractive index increases from a light incident side to the substrate,
• wherein the intermediate layer has a first layer closest to the substrate, and a second layer formed on the first layer, and
• wherein when a refractive index of the first layer to light having a wavelength of 550 nm is denoted by n1, a refractive index of the second layer to the light having the wavelength of 550 nm is denoted by n2, and a refractive index of the substrate to the light having the wavelength of 550 nm is denoted by ns, the following conditional equations are satisfied:
• 1.75≦ns≦2.20;
• n1<n2;
• and
• n1<ns.

An optical apparatus comprising an optical system that includes an optical element, the optical element including a substrate that is transparent to light having a used wavelength region and an antireflection coating formed on the substrate,

• wherein the antireflection coating includes an intermediate layer formed on the substrate, and an undulation layer that is formed on the intermediate layer and has a plurality of convexes arrayed with spacing shorter than a shortest wavelength in the used wavelength region,
• wherein the undulation layer has a portion where refractive index increases from a light incident side to the substrate,
• wherein the intermediate layer has a first layer closest to the substrate, and a second layer formed on the first layer, and
• wherein when a refractive index of the first layer to light having a wavelength of 550 nm is denoted by n1, a refractive index of the second layer to the light having the wavelength of 550 nm is denoted by n2, and a refractive index of the substrate to the light having the wavelength of 550 nm is denoted by ns, the following conditional equations are satisfied:
• 1.75≦ns≦2.20;
• n1<n2;
• and
• n1<ns.