Grism

A grism comprising:

• a prism the vertex angle of which can be varied;
• a VPH grating being a diffraction grating and disposed substantially orthogonal with respect to a side including the vertex angle of said prism;
• and
• holding substrates for holding said VPH grating;
• said VPH grating being rotated around an axis being substantially orthogonal to the side of said prism, whereby a slope angle defined by said VPH grating and a predetermined plane is varied.

A grism comprising:

• a first prism the vertex angle of which can be varied;
• a second prism the vertex angle of which can be varied;
• and
• a VPH grating being a diffraction grating;
• said grism being formed by sandwiching said VPH grating in between said first prism and said second prism so as to oppose the vertex angle of said first prism to the vertex angle of said second prism;
• light being input from the outside through a surface of said first prism, the light input inside said first prism being input to said VPH grating after transmitting through said first prism, the light passed through said VPH grating being input into said second prism, and the light transmitted through said second prism being output to the outside from the surface of said second prism.