Observation optical system and display apparatus

1. An optical system configured to guide a light beam from a display element, the optical system comprising:

a positive lens;

a negative lens; and

an optical element having a transmission surface, an optical surface, and a reflection and transmission surface,

wherein the light beam from the display element;

passes through the positive lens, the negative lens, and then the transmission surface;

then reflects on the reflection and transmission surface and then on the optical surface;

then passes through the reflection and transmission surface; and

then travels to an exit pupil, and

wherein the following inequalities are satisfied:

0.28<nn-np<0.60

50.26≤vp-vn<80

where np and nn are refractive indices of the positive lens and the negative lens for the d-line, respectively, and vp and vn are Abbe numbers of the positive lens and the negative lens for the d-line, respectively.

2. The optical system according to claim 1 , wherein the optical system guides the light beam to the exit pupil without forming an intermediate image.

3. The optical system according to claim 1 , wherein the following conditional expressions are satisfied:

0.0<(φ p/np+φn/nn )/Φ<0.6

−0.03<(φ p/vp+φn/vn )/Φ<0.00

where φp and φn are powers of the positive lens and the negative lens, respectively, and Φ is a power of the optical system.

4. The optical system according to claim 1 , wherein the positive lens and the negative lens are cemented to each other.

5. The optical system according to claim 1 , wherein the negative lens is disposed between the positive lens and the optical element.

6. A display apparatus comprising:

a display element; and

an optical system according to claim 1 .

7. The optical system according to claim 1 , wherein the following inequality is satisfied:

0.4213≤nn-np<0.60.

8. The optical system according to claim 1 , wherein the number of lenses arranged between the display element and the transmission surface is three or less.