Light waves will bend when passing from one transparent substance to another unless:

Unless the light ray strikes the medium at an angle of 90°

The phenomenon of bending of light when going from one medium to another is referred to as refraction. However, refraction is only possible when the light ray is not perpendicular to the medium. If the light is perpendicular, it passes through without refracting.

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B) raising the temperature and reducing the volume of the gas.|-/