Why does the camera lens look colored?

The lenses of advanced cameras look mostly blue and purple. Do you know why? In fact, this is because a magnesium fluoride film with a thickness of about 100 nm was plated in front of the lens.

If you want to know the function of this film on the lens surface, you have to start with the reflection and refraction of light on the lens. From the point of view of energy, for any lens, the energy of light can not completely pass through the interface, and there will always be some reflection from the interface. Whether light enters the glass from air or air enters the glass from glass, the reflected light energy only accounts for about 4% of the incident light energy.

In order to get better photographic effect, multiple lenses are often used inside the camera. In this way, the loss of light energy will reach 50% or even more. Some complex optical systems composed of 20 lenses will lose more than 90% of light energy! These reflected lights become harmful background stray light, which seriously affects the imaging quality of the camera.

So, is there any way to reduce the reflection of light on the lens surface? The method is to plate a magnesium fluoride film with a specific thickness on the lens. The existence of this film makes the reflected light destructively interfere, which can reduce the reflectivity of the lens surface and reduce the loss of light energy. This kind of film is usually called "antireflection film". In fact, by using this single-layer antireflection film, the light reflectivity can be reduced to about 65438 0.2%.

As we all know, sunlight contains many different colors, and the antireflection film on the lens can't make them all destructively interfere, but can only reduce the reflected light intensity of individual wavelengths to the minimum, and can only weaken the reflected light of other similar wavelengths to a certain extent.

For example, the antireflection film of a camera lens generally selects the yellow-green light that is most sensitive to human eyes for destructive interference, so that the reflected light intensity is weakened, so that the camera lens presents a blue-purple color with complementary yellow-green light.

Summary: In practical application, there are sometimes completely opposite requirements, such as reducing the transmittance of light as much as possible and improving the reflectivity of light. Such a thin film is called a "highly reflective film". If only one layer of thin film can't greatly improve the reflectivity, multi-layer thin film can be used, and the highest light intensity reflectivity can reach above 99%.