1. Selecting Glass Types for Inner Engraving

Recommended Glass Types:

Borosilicate Glass

K9 Optical Glass

Quartz Glass

Advantages of the Recommended Glass Types:

They offer uniform composition and low impurity content, which reduces UV laser energy scattering and ensures dot-matrix accuracy and structural consistency for 3D engraving, helping prevent uneven depth and blurred edges;

they feature low internal stress and excellent thermal stability, which helps offset localized thermal stress generated by concentrated energy during engraving, effectively reducing the risk of cracking or embrittlement and improving process stability;

they provide high transparency with minimal base color, enabling clear presentation of 3D inner-engraving layers and fine details without color interference, resulting in strong visual clarity and depth;

and they are well-suited to processing requirements, as their surfaces can be easily polished flat to meet positioning accuracy, and their defect-free structure (free of bubbles, scratches, and similar flaws) helps maintain a continuous, stable laser focusing path and prevents deformation of the 3D structure.

2. A Simple Method to Measure the Refractive Index of a Glass Block

2.1 Laser Pointer + Protractor Method

Materials Required: A regular glass block, a laser pointer (a household red or green laser is acceptable), white paper, a pencil, a ruler, a protractor, and tape (for securing the glass block).

Core Principle: Snell’s law (n = sinθ₁ / sinθ₂).

Procedure:

• Secure the glass block on the white paper, trace its outline, draw the normal line through the incidence point, and mark the 30°/45°/60° incident-angle guides.
• Aim the laser pointer at the incidence point along each marked incident angle and trace the refracted beam path.
• Measure the refracted angle θ₂ and calculate sinθ₁/sinθ₂, then repeat the measurement multiple times and use the average value.

2.2 Apparent Depth Method

Materials Required: A regular glass block, a coin (or a small object), a ruler, and white paper.

Core Principle: n = actual depth (d₁) / apparent depth (d₂) (when viewed vertically, the apparent depth becomes shallower due to refraction).

Procedure:

• Measure the actual thickness d₁ of the glass block (vertical dimension).
• Place the glass block over the coin, observe vertically from directly above, and measure the “apparent depth d₂” (the distance from the top surface of the glass to the apparent image of the coin).
• Calculate n = d₁/d₂, repeat three times, and take the average value.

2.3 Parallax Method (Based on Imaging Differences, No Angle Calculation Required)

Materials Required: A regular glass block, a coin, a ruler, white paper, and a pencil.

Core Principle: n = d / (d − h) (where d is the glass thickness, and h is the vertical distance between the reflected image and the refracted image).

Procedure:

• Measure the glass block thickness d.
• Place the glass block over the coin, observe horizontally from the side, and mark the coin’s “reflected image” (reflected by the top surface) and “refracted image” (seen through the glass due to refraction).
• Measure the vertical distance h between the two images, then substitute the values into the formula to calculate n.