69: Learn to make a telescope 2

“Oh, Zi Xuan, didn't you say your deskmate was coming over to learn how to make a Magnifying Glass with me? Why didn't he come?”

Xu Bai asked, puzzled.

“Brother Xu, we have a pass from the Homeroom Teacher. He’s teamed up with someone else, so he can’t leave school. He can only come after school.”

“Oh, then sit down, both of you, and take out the best Magnifying Glass you’ve practiced making for me to see. If it’s good, I’ll teach you about the Telescope today.”

“Okay.” “Okay.” “Okay.”

The three of them took chairs, found a spot at the counter, sat down, and placed a Magnifying Glass on the counter.

Xu Bai inspected the Magnifying Glasses made by Zhao Zixuan and the other two one by one. After confirming there were no issues, he began his little class.

First, on the small blackboard, he drew four things with white chalk: a simple stick-figure tree (△ + |, combined), an objective lens and an eyepiece (represented by two ovals, with the objective lens being a large oval and the eyepiece a small oval), and finally, a simple stick-figure eye.

Xu Bai planned to first teach them the principle of magnification for the Keplerian Telescope, which is very classic and consists of two convex lenses plus an erecting system.

Then he used yellow chalk to draw the light path diagram.

“Our Telescope is composed of two convex lenses. The objective lens has a longer focal length, and the eyepiece has a shorter focal length.”

“This is a tree. When sunlight shines on it, it reflects outwards. Through the reflected light, we see it. When parallel light rays from a distant object pass through the objective lens, it forms an inverted, diminished real image beyond the focus, very close to the focus. I’ll draw it for you with a solid line here.”

With that, Xu Bai used a red solid chalk line to draw a small, inverted, diminished tree. This small tree was slightly in front of the eyepiece.

“Look at this light path diagram. The front focus of the eyepiece coincides with the focus of the objective lens. The real image is located between the eyepiece and its focus, very close to the focus.”

“Then, the eyepiece will use this real image as an object to form a magnified virtual image. This image, compared to the first image, is an ‘upright,’ magnified virtual image.”

Immediately after, Xu Bai used a red dashed chalk line to draw an ‘upright,’ magnified small tree. Xu Bai drew this small tree behind the objective lens.

“Why do our eyes ultimately see an upright, magnified image? It’s because I’ve added an erecting system in the middle. This is a prism. A prism can invert an inverted image both vertically and horizontally, thereby turning it into an upright image.”

“The light rays entering the eye appear to come from the magnified virtual image, increasing the angle of view and making the observer feel that the object is closer and clearer. This is a Telescope.”

Xu Bai finished drawing the Telescope’s light path diagram and explained each part. Finally, he said, “The magnification of a Telescope is equal to the ratio of the objective lens’s focal length to the eyepiece’s focal length. You can use this to make Telescopes with different magnification effects, which is also why the Telescope I made can be rotated and adjusted.”

Fang Mu listened carefully to Xu Bai’s explanation and a question arose in his mind. He raised his hand.

“Fang Mu, do you have any questions?”

“Teacher Xu, does a Magnifying Glass always require two convex lenses? Using two convex lenses would form an inverted image, and a prism would also need to be added. What if we used one convex lens and one concave lens?”

Good students are like this; they can generalize from one example. Xu Bai was very satisfied with this.