# OpticalBench, Lens and Ray

> OpticalBench is manim-physics' Lens and Ray: lenses on one horizontal axis, and rays traced through them from left to right.

Page: https://fframes-kit.pages.dev/docs/physics/optical-bench-lens-and-ray/
Cargo feature: `physics` (`fframes-kit = { version = "..", features = ["physics"] }`)

`OpticalBench` is manim-physics' `Lens` and `Ray`: `lenses` on one horizontal axis, and
`rays` traced through them from left to right. `Lens::thin(x, focal)` is a thin lens,
`Lens::thick(x, r1, r2, index, thickness)` a real one from its radii, refractive index and
thickness (`focal_length()` is the lensmaker's equation). Radii are positive when the centre
of curvature is to the right, so a biconvex lens has `r1 > 0` and `r2 < 0`; a flat side is
`f64::INFINITY`. The rays are traced with paraxial optics, surface by surface, and a ray that
meets a lens outside its `height` stops there. Each lens shows its front and back focal
points, `F` and `F'`, found by tracing a parallel ray, so a thick lens's sit at its focal
distances from its surfaces. Gallery scenes: `OpticalBench`, `Lens`.

Gallery clip of `OpticalBench`: https://fframes-kit.pages.dev/media/scenes/OpticalBench-a036ca3a.mp4

Gallery clip of `Lens`: https://fframes-kit.pages.dev/media/scenes/Lens-3dcda149.mp4

Parallel rays focus through a thin lens:

```rust
use fframes_kit::prelude::*;

fn bench<'a>(ui: &mut Ui<'a, '_, '_>) -> Svgr<'a> {
    let rays = (0..7)
        .map(|i| Ray::new((-6.0, -0.9 + 0.3 * f64::from(i)), (1.0, 0.0)))
        .collect();
    ui.show(OpticalBench {
        lenses: vec![Lens::thin(0.0, 2.5)],
        rays,
        x_range: (-6.5, 6.5),
        delay: 0.5,
        enter: Some(Create::default().into()),
        ..ui.defaults()
    })
}
```

For a thick lens followed by a diverging thin lens:

```rust
use fframes_kit::prelude::*;

fn bench<'a>(ui: &mut Ui<'a, '_, '_>) -> Svgr<'a> {
    let rays = (0..7)
        .map(|i| Ray::new((-6.8, -0.9 + 0.3 * f64::from(i)), (1.0, 0.0)))
        .collect();
    ui.show(OpticalBench {
        lenses: vec![Lens::thick(-3.0, 2.4, -2.4, 1.5, 0.5), Lens::thin(2.6, -2.0)],
        rays,
        delay: 0.5,
        ..ui.defaults()
    })
}
```

## API

Components are plain structs. Set the fields you need and fill the rest with `..ui.defaults()`: the defaults below come from the current theme (`theme.` values are fields of `Theme`).

### OpticalBench

Manim-physics' `Lens` and `Ray` on one axis: `lenses` on a horizontal axis at height `axis_y` from `x_range.0` to `x_range.1` (units of `ui.space`), and `rays` traced through them to `x_range.1`. With `focal_points` each lens's front and back focal points are dots labelled `F` and `F'`, found by tracing a parallel ray through the lens, so for a thick lens they sit at its focal distances from its surfaces, not from its middle. The rays draw on one after another from `delay` seconds after the local start (`ray_time` each, `stagger` apart). The axis and lenses draw with `enter`, the rays and labels with `parts_enter`.

| Field | Type | Default | Description |
| --- | --- | --- | --- |
| `lenses` | `Vec<Lens>` | `Vec::new()` | The lenses on the axis, each with its position and shape. |
| `rays` | `Vec<Ray>` | `Vec::new()` | The rays traced through the lenses to the right end of `x_range`. |
| `axis_y` | `f64` | `0.0` | Height of the horizontal axis, in units of `ui.space`. |
| `x_range` | `(f64, f64)` | `(-7.0, 7.0)` | Left and right ends of the axis in units of `ui.space`; rays are traced up to the right end. |
| `focal_points` | `bool` | `true` | Marks each lens's front and back focal points with a dot. |
| `labels` | `bool` | `true` | Writes `F` and `F'` under the focal points. |
| `delay` | `f32` | `0.0` | Seconds after the local start before the first ray begins to draw. |
| `ray_time` | `f32` | `0.8` | Seconds each ray takes to draw. |
| `stagger` | `f32` | `0.25` | Seconds between the start of one ray and the next. |
| `lens_color` | `Color` | `theme.colors.text` | Colour of the lenses, the focal points and their labels. |
| `ray_color` | `Color` | `theme.colors.highlight` | Colour of the rays. |
| `guide_color` | `Color` | `theme.colors.muted` | Colour of the dashed axis. |
| `stroke_width` | `f32` | `theme.stroke.regular` | Width of the lens outlines in pixels; rays use the theme's thin width. |
| `enter` | `Option<EnterAnim>` | `None` | How the axis and lenses draw in; `None` shows them from the start. |
| `exit` | `Option<ExitAnim>` | `None` | How the axis and lenses leave at the end of the container; `None` stays. |
| `parts_enter` | `Option<FadeIn>` | `None` | How the rays and labels fade in; `None` for none. |
| `parts_exit` | `Option<FadeOut>` | `None` | How the rays and labels fade out before the end; `None` for none. |

Feature `physics` · [Source](https://github.com/khanhthanhdev/fframes-kit/blob/main/fframes-kit/src/kits/physics/lens.rs#L356)